i hated being 15 and wanted to die too, or at least have something change. now that i can look back, i can identify what, specifically, gets "better:"

-at 15 you have almost no control over your schedule, including sleep. the school system demands a sleep/wake cycle that's extremely unnatural and difficult to keep even for normal teenagers, and amounts to constant sleep deprivation (a form of torture) for nightowl teenagers. constant sleep deprivation or even just chronic disregulation fucks with your entire body, your hormones, and your mental health. it makes everything worse.

-as an adult, a lot of jobs do expect you to be a morning person, but at least your body is more likely to cooperate with a 9-5 work/sleep schedule, and you're also more free to take second shift or night shift work that suits you. as an adult you need less sleep than teenagers and have fewer authority figures in your life to get in the way of when and how much you nap.

-at 15 if you attend any school other than homeschooling, you're basically in jail that's also hell. everyone is a similar age to you and going through similar, or worse, problems, and they're going to take it out on everyone around them. you have very little ability to get away from bullies, and you're just fucked when your bullies are your teachers, your parents, your parents' friends. if you're attending homeschooling, you trade an inescapably crowded and stressful social situation for an inescapably isolated social situation, one where your parents have absolute authority over every part of your life. a solitary prison is even worse than crowded one.

-as an adult, a dysfunctional and abusive workplace has a huge impact on your mental and physical health, and many people do die or kill themselves because of a degrading and terrible job. however, as an adult, you're likely to have more power to chose your workplace, it's likely to be a shorter and less rigorous day than highschool+homework, and you get paid for it. plus, a good job is extremely empowering and satisfying.

-at 15 your body is growing and you need large, regular, balanced meals. lots of 15 year olds skip breakfast and have shitty lunches, because school is terrible. lots of 15 year olds have food insecurity or are on diets, either chosen or imposed, and this makes them depressed and emotionally volatile. at 15, you probably don't have very much positive control over your meals, since your parents are in charge of groceries and meal prep. just negative control, by skipping or refusing. replacing meals with soda or candy is also a problem-- your body craves a big hit of sugar at all times, because you're growing, but replacing meals and sleep with soda just leads to more disregulation. teens can easily get into really dysfunctional cycles this way.

-as an adult you have more time to eat and more control over what you eat. you have your own salary and make your own shopping lists. as an adult you're also not going through growth spurts anymore: your body grows and maintains itself through its whole life, but it's steadier. you work out what you like and you know how to get it.

IN CONCLUSION: at 15 you're depressed for a lot of good reasons. if someone has a grueling and unrewarding schedule that traps them with other miserable people all day, subject to hostile authority figures with no escape, and they're expected to do lots of work with few rewards and frequent punishment, they're going to be miserable. this is further compounded when a person has a rapidly growing body that can't possibly get a healthy amount of nourishment and rest. these are the conditions that reliably cause depression.

as an adult, we're telling you "it gets better" because in about four years you will be free of so much of what's making you miserable right now. to eat food you like, to rest as much as you need, to work a paying job or chase your own dreams, to decide what kind of life you want to live. it might not sound like much, but it's everything.