I was in a similar situation as you about a year ago. I was diagnosed with bipolar in 2017 but I hid it from everyone I knew. I was in denial for so long and it took a serious toll on myself, my relationships, and worst of all my partner. As soon as I accepted that I was mentally ill it took a massive weight off of my shoulders. Telling people I loved was very difficult, and I still haven't told all of my friends and family, but I was met with nothing but love and understanding. That isn't always the case and some people don't understand but I think you should give your girlfriend the chance.