It doesn't sound like the most healthy relationship right now. Especially if he gets jealous when you go out. I was wondering if that was the reason that you didn't want to go alone. With his unhealthy moods, it sounds like it is really rough for you. Maybe you need to sit down and talk about it with him. Can you do that, or does he become defensive?
What is an MI? Sorry I don't know.
It sounds like he is really taking advantage of your good nature. That isn't right. You can only live with so much and it will get you much more depressed. So I would take the health issue into consideration and really think about the relationship. I am sorry he wont get help. You can't help somebody that doesn't want it. I know you love him, but seriously think about what it is doing to you.
Good luck, keep me posted.