So much good advice in this. You are doing well in learning from your experience. I'm 35teen and am still struggling with some of these, even though I have the experience to back up what is stated here.

I completely changed my career at 29 and am doing better, but I still fail at comparing myself to others and think about how much better I could be if I had found my current career path out of HS.

I still hurt every time I think about the friends that have chosen to leave me out, and think about what I must have done to drive them away. Hint: it's not anything you did. Unless you obviously did something very shitty, there's nothing to figure out. Like OP said, it just happens.

Even though I suffer from depression and anxiety, and have been able to manage other conditions, I still have trouble helping others who have mental illness even though I (sometimes) know what they are going through. Some things are hard to overcome. The important part is to try.

But one thing that is true and you don't need to think twice about is: DON'T DO HEROIN. I'll never get back my friends I lost to it. Just please god, don't fucking do it.

Big ups OP. Best wishes to you and your future.