BFA, you say you know the ups and downs that come along with a mental illness, so you must know how hard it is to come up from a down time, even when you have professional help and medication. You know how irrational and scary those times can be for everyone involved. Now think of it this way, do you really want to be on the receiving end of someones emotions who is going through this who refuses to get help?
I've been in a very similar situation, and I'm bipolar too. It's really easy for a situation like this to trigger very bad times for yourself. You need to ask yourself if you logically think that this is healthy for you. Keep in mind hes only 15. I know you want to think that he's the one thats meant for you, but do you think he's mature enough for a serious relationship?
Talk to your doctor, and make the decision that is best for you and your future. You're a really nice girl and none of us want to hear about you getting hurt because you fell in love with the wrong person.