Soooo...your girlfriend had 20 one night stands BEFORE you met her. What do you have to be hurt about? She wasn't with you, so she didn't cheat on you. Was she with someone else or something at the time of the one night stands? If that's the case, it's different, because she has a pattern of infidelity, but if she was single, who cares?
People cheat, with or without bipolar. It's not fair at all to give her a hard time for something that happened before you. Obviously she trusted you enough to tell you, it's not fair for you to throw it back in her face.
It's time to put on your big girl/boy panties on and suck it up.