a lot of people would shoot you for this. but dont worry i am not one of them. be careful out there tho
i think the first thing i would say is that instead of watching a video, you would need to do play a full solavellan playthrough of the game if you do want to genuinely understand the relationship and why it is so beloved. im not sure which compilation you watched, but even one that includes all their conversations (rather than just the cutscenes, of which there are very few) cannot do the relationship justice. so much of understanding solas as a character and how he loves people, by extension, is wrapped up in how he reacts to the world at large, its people, its history, its institutions, and its metaphysics. assuming you're new to DA and wouldn't know this, solas's romance in inquisition is the shortest, most sparse romance in the game, and was added later in development. as a result, much of his essential characterization happens outside the bounds of romance content, but still adds deeper meaning, context, and depth to the relationship. even in terms of romance specific content, some of my favorite content occurs in banter that probably was not included in the video you watched. the solas romance is less a standalone love story, in the way many of the romances are, and more of a big juicy delicious cherry on top that helps you better understand the overall dragon age solas plot/cake you're eating.
theres a couple non-romance specific scenes that shed significant light on solas & mythal's dynamic from his perspective that i am not sure if you have seen and honestly i wouldnt recommend watching them because, again, i think you should just play inquisition and experience them in the proper context. but solas's companion personal quest is directly about his corruption at the hands of mythal, though we didn't know that until veilguard came out and contextualized it. and this quest pretty explicitly demonstrates how he feels about what she did to him: rage, beyond forgiveness, deserving of death. he also comments on her at the temple of mythal, and his comments are mostly neutral but verging on judgemental, and do illuminate that while he may have loved her, he certainly did not trust her. it is he who first clarifies that she was a goddess of vengeance, rather than justice. which i cant think about too long or else i'll get angry that they ret-conned it to benevolence -> retribution or whatever the fuck and erased the anders/justice/vengeance parallel... anyway
but i think more telling is his absolute refusal to drink from the well if asked, and most telling; how he fears for an inquisitor who drank.
he specifically calls mythal dangerous, arrogant, and fickle, absolutely refuses to submit to her will once again via the drinking of the well, and begs an inquisitor he loves not to do the same lest she suffer the same fate. he loves mythal, of course, but he also fears her. he is critical of her behavior and wary of her motivations. his love for her exists alongside his recognition of what she was.
another fairly vital bit of information is how according to trespasser (cole banter), solas used to wear mythal vallaslin until he burnt it off his own face when he developed his vallaslin removal spell. its how he got the little scar above his eyebrow. meaning, if vallaslin were slave markings, that solas was effectively enslaved to her. this is... pretty important context, obviously. but we never find out what it might have been like for him. veilguard.... didnt forget but rather deliberately ignored this because it wasnt willing to interrogate the issue of slavery which had been vital to solas as the leader of a slave rebellion. ugh. anyway.
this leads into my next point which is that veilguard really drastically changes solas's motivations to be far more mythal-centric than what was set up in inquisition/trespasser. we always knew something was up with them, and people always wondered if they might have been lovers, but veilguard goes in on this idea in a way that many people would actually call out-of-character compared to how he behaves in inquisition. veilguard itself though does present their relationship as rather complex though, in my opinion its one of the best parts of the game. the two moments that i chew on most frequently are the letter from felassan in mythal's weird little dragon pit that reveals how he made that island for her but locked it away when she was killed. and my ultimate fave is how she reveals that in the literal thousands of years she has been sitting there alone since her murder, many of which he was alive and fighting a rebellion partly in her name, and in the 12 years since he woke up from uthenera, he never went to visit her. not once. its giving jane eyre and i fucking love it. in this same conversation, she also says that when he killed flemeth, he wept. this, i think, is the crux of how he feels about her. he can barely look at her. he resents her. he will use her like he did anyone else. he loves her. he feels lost without her. he will never forgive her. he misses her. all of these things are true at once, and mythal seems to feel similarly; she loathes him. she understands him better than anyone. she resents him for betraying her and abandoning her. she calls him a pathetic little crybaby pussy ass bitch. she loves him.
i dont think anything you said in your message is necessarily wrong. i do think he loves mythal still. i think he always will. i think mythal is valid when she says that they have a bond that no one will ever understand. i agree he is protective over her. i also interpret their relationship as romantic though a lot of people do not. i just love drama. but i think you are misinterpreting his reluctance to be with lavellan as coming from his attachment to mythal as a person, rather than his attachment to his duty to what mythal represents - the world he ruined, everything he's ever done wrong. to say that solas would actually consciously choose mythal over lavellan if they were the final two contestants on the bachelorette is honestly, absurd. sorry. because actually he would choose neither, he would dramatically let the rose fall to the ground and run off to restore the elven people while chris hansen (felassan) dramatically runs after him. both women are secondary to him when it comes to the good of the entire world, and fixing what he broke. he has had plenty of moments to choose mythal and run away with her if he wanted. he has literally had her bertha-ing out in his crossroads attic for 10 years. he also literally does kill her via flemeth. which isnt to say that he wouldn't kill lavellan if forced to, i think he would. but the point here is that its not mythal vs. lavellan. its mythal vs. the world, and lavellan vs. the world. he should have chosen the world over mythal. he didnt. he created the blight instead. he destroyed everything. he cannot make the same mistake again, so he will choose the world every. single. time.
regardless, every time solas turns away from lavellan in the romance, he is not thinking "i wish you were her". he is thinking "if i do this to you, i have become her". prioritizing his own desires over the good of the world, stringing her along, using her as a tool to do his bidding (getting the orb back), are all things mythal did to him. he told her he would follow her anywhere. and when he begins to realize that lavellan would follow him anywhere (as she says in veilguard), he freaks out and has to end it. he knows he will have to continue to kill and cause destruction to bring his world back, so if he did allow her to join him in walking the dinan'shiral, or did anything other than break her heart and leave her, he would be corrupting her the way mythal corrupted him; a weapon to achieve his goal. but he refuses. in his mind, he already destroyed the world for love once; at mythal's behest. if he abandons the world for lavellan, he is destroying the world for love again, and making her an accomplice. so, every time he leaves her it is an act of love.
the way the inquisitor is the driving force of their romance is partly just... gameplay lol but its also consistent with the overarching theme of consent in a relationship that is fundamentally unethical and unequal. lavellan has to be the initiator or else solas becomes a predator. some would say he is anyway lol, but its clear much of the writing was designed to avoid this with the way he is constantly denying himself, backing away, trying not to give in. it might have been juicy, but for him to knowingly romantically and sexually pursue a young woman 10,000 years younger while lying to her about his identity and using her for his plans would make him an entirely different character. a character that would be a hit on romantasy booktok, but not solas. consent and ethics are so central to not only the relationship thematically, but to solas himself, and some of that is because of mythal and the inequality of their own past dynamic. solas is so passive in the romance not because he doesnt like this weird clingy bitch who wont leave him alone, but because he does not want to recreate the same dynamic that corrupted him into pride and uhhhh literally destroyed the world. i'll leave you with another essential quote that you may not have encountered yet:
Cole: It isn’t abuse if I ask!
Solas: Not always true.
in trespasser, solas's duty to bring down the veil was more unambiguously to the elven people and the alleviation of his own crushing guilt, while mythal was collateral damage in his way and he used her like he would use anyone else (including lavellan loool) as a tool to achieve his goals. we see this when he kills flemeth and takes mythal's power. in veilguard they had to obscure this slightly to make him "less sympathetic", to use the devs own words. and they did this by shifting the crux of his motivations to mythal. i dont think his lap dog devotion is out of character, i adore it, but i hate that it came at the expense of his more complex and sympathetic motivations of saving the elven people and spirits from the damage of the veil. as a result, when looking at his behavior in the context of inquisition + trespasser + veilguard, i interpret it as mythal being symbolic of the destruction of the world at his hands. and not to toot my own horn but trick's interpretation that they shared on bluesky does support this, when they said that to solas, mythal represents the past and lavellan represents the future. ive written about his statement that it was all for mythal, and the tldr is that i think it is also supposed to be interpreted as symbolic and reflective of his psyche. but even if he did do it all for her, i dont think that necessarily negates his relationship with lavellan. he needs mythal to break the cognitive dissonance, alleviate his guilt, and release him, because she is the source of all of those things in the first place. lavellan could never break them because she is frankly irrelevant to those things. he is so caught up in his sunk-cost fallacy that he feels the only way is through. lavellan may not be able to break the hold the past has on him because she is separate from it, but she can offer him another path once it has been broken, a fork in the road he thought was straight; her, their future.
i think to say solas's heart is not free is a misunderstanding. he denies his heart's desire over and over, we see this clearly in the letter he sends to lavellan in veilguard that expresses how badly he wanted to put down his burden and stay with her. in his expressed reluctance to leave her in crestwood, how he refuses to lie and tell her it meant nothing. in "no matter what happens, i want you to know that what we had was real". his indulgent final kiss in trespasser. in "i will never forget you". its especially apt that you worded it this way and that vhenan means "my heart". if anything, his heart is the most free part of him. it is everything else that belongs to mythal: his body, created at her command. his path of destruction and ruin, which she set him on. his purpose, which she distorted from wisdom to pride. she, then, is the only one who can give it all back to him. and as soon as she does, he is free to prioritize his heart. and he quite literally does.
tldr; play inquisition <3