My boyfriend and I have been together for almost 5 years now. When we first started dating, he was very honest with me about his mental health problems, and he was in therapy and on medication. After two years together, we moved to a new city. Since then, everything's gone downhill. He started getting really jealous of any time I spent with new friends. After a while, I stopped trying to make friends at all because it wasn't worth the grief at home. It escalated to the point that we were barely talking to each other.
We finally split up and I tried seeing someone new, but he spent the next 6 weeks threatening to kill himself, hurting himself in front of me, showing up in the middle of the night in hysterics, and threatening to hurt the guy I was seeing (he was convinced I started seeing him before we broke up). I finally gave up and decided to try it again. It got better for a little while, but then he quit therapy, quit his meds (he's an artist and claims he can't be creative while on psych drugs), and now he even gets jealous over my professor/mentor at school (I don't bother trying to make real friends). Every time I try to end it, he tells me how his life might as well be over, and he won't make it without me.
I'm completely miserable. But if we break up, we'll have to live together for a least a few weeks before one of us can move out. I know he'll spend every moment trying to change my mind, with begging and threats, and I don't know if I'm strong enough to not give in. And I'm terrified that he'll try to kill himself again. My family loves him, so they're not supportive. I don't have any friends in the city I can stay with. I'm trapped.

I think you have to convince him to start therapy again.. and maybe talk to his parents but you have to make sure that they won't let him know that you talked to them.. If he feels that you've betrayed him in that sense then he might hurt himself again..
Many guys throw death threats at their ex's, it doesn't necessarily mean they will go for it but you can never risk it with these delicate situations.. If you intend on breaking up with him - which seems to be on your mind then you need to make sure he has a stable environment to fall back on.. I realize it's a very hard situation for you but you only have one life and its unfair for you to have to live it in fear because of someone else who is being selfish.
So I think the first step for you is to get him to start therapy.. If you are intending on breaking up make sure you distance yourself from him first.. Don't make love, try to avoid kissing and hugging e.t.c... If you throw a bomb like that to a person who is unstable it will surely upset him and then god knows what he could do to himself.So make sure you prepare him for it somehow.. you can't rush into it.
I'm sorry that's all the advice I can offer for now - I'm out of ideas.

You don't deserve that. He is controlling your actions through fear of him hurting himself.

I know its hard, but you need to get out of the relationship. His actions aren't really your responsibility. Can you talk to his family about it so that someone can keep an eye on him as you disappear?

Start him on therapy. NOW.
Get him some help, then get him some friends, and something other than you to rely on.
Then break up with him, but stay close to him as a friend.
He will need something to help him and as long as you don't get too close you can still have a good bond together as friends as he becomes more stable.
The more you keep giving in to his threats the more they will happen and the worse they will get. Be compassionate, but also stay strong in your decision.

Lady get the hell away from him right now, and stay away. You don't have to be a hostage. No Excuses.