I don't know why heart kind of dropped reading this. I guess because I really relate.
I'm probably a bit older than you. There was a time in my life when I tried to replace my shit family with a "family" of close friends. I invested time, energy, attention, dutiful care, and even money into my friends to be there for them in times of need, celebration, and milestones. Despite all my mental issues, I fought through myself to protect those friendships, but when shit got real for me and my life began to fall apart, those friendships all fell apart so quickly and permanently and quite honestly, I think I'm still broken by the conditional, temporary nature of most friendships and relationships.
If it's okay, I'd like to offer some sincere advice. Do your best to remember that 90 percent of the people that come through your life will probably not be there forever. That's not to discourage you or to make you feel bad. It's just to say that the best way to cope with disappointment is to change your expectations out of people, to enjoy the moments you have with people, and don't hold on too hard. Don't expect too much and you won't feel as let down. Granted, for some of us who are already so isolated and needing support, that's easier said than done. I feel for you, OP.