It's super weird that this topic came up. I was going to make one a couple days ago when I was feeling depressed, then figured it wouldn't gain much momentum so I decided against it.
I don't know what I have, but I have bouts of extreme sadness, thinking back on all the people I've cared about that aren't in my life anymore (friends, significant others). I would never self-harm though, or do worse. It's just upset moments, not full on "my life is over I can't take it anymore" moments, no offense to those who get them.
My friends are few and far between, but when I'm around them, laughing and having a good time, all that "depression" fades. Then as soon as they're gone, it comes back. I also have a weird phobia of eventually losing these few friends over some kind of argument or drifting apart or whatever. It keeps me up, thinking to myself that someday this person that means so much to me is going to leave my life and I'll miss them.
Getting myself emotional just by typing this stuff so I'll stop it here.