My boyfriend has cyclothmia, which is a milder type of bipolar disorder, but I can still relate to what you're going through (even the part about his mom not liking you).
All that you can do is be there for him, but please please please look after yourself as well. Think about it this way: if are not positive and stable yourself, how can you help him?
My boyfriend and I did break up once, but we realized that we still had strong feelings for each other, so I made a conscious decision to get back together with him knowing that it would not ever be smooth sailing. So I understand how painful the breakups can be.
In the end, it is your choice to stay with him, and if you do, please make sure that you keep in touch with your friends and go out and have fun.
Forgive me for this cliche, but you won't be 18 forever. It is important that you also enjoy your life.