What am i doing wrong?

I need help I feel like I did something wrong my girlfriend she knew I was with this guy and she told me he was cheating and when I asked him about it and he said he wasnt and when I told her that he wasnt she dumped me because I didnt listen to her and I feel like I did something worng and I dont know what it is I need some help please. I think she only wanted me to her self because she is lez and I am bi.