Honestly I think you need to tell her about your Bipolar , that way she can do research to get an understanding of Bipolar disorder. I'm pretty sure you have been Bipolar long before you got the "diagnosis' or "label" only change now is that your getting help.

You have been with her over a year, she should know... I'm sure she will look back and be able to understand better why you have acted the way you have in the past in some situations.

Good luck with however you decide to handle things.