Hey jammin.....it sounds like he really feeds off your emotions.
I have a very good friend and children who are bipolar and I know from experience how easy it is to get caught up in the roller coaster ride that the individual is on. It's very hard for us to not ride that roller coaster with our loved ones, but it is also not healthy for us to ride it.
It's very possible that he's at a point where he doesn't know how to handle your lows anymore, or even that he's feeling some depression as well and not feeling in control of his world. It's really hard to see those we love and care about going up and down and all around and it can be tiring and stressful as well.
Maybe finding some books for you both to read on the ups and downs of Bipolar will help you and him to understand what happens and why and how to deal with those things most effectively. Searching Amazon.com will bring up a number of different books on the subject.
I don't know that your boyfriend is wrong...I think he is totally caught up in your swings and is feeling powerless to help you. That can make one feel very low when we know we can't help those we love.
I wish you both well and hope you can find some middle ground and some help in dealing with this issue.

sabby