Cheating girlfriend

Justyna
In two weeks it will be two years together and we have a great relationship but.. yesterday I found out she cheated on me at the beginning of our relationship. When we were 3 months together and already serious. I was invested in her from the first day. I had to fight for her because she still had feelings for another girl. I knew it but she always said she is over her. But she did it with her. She hasnt seen her since that happenned but I dont know if I should forgive but I really want to.