I Saw My 18-Yr-Old Vírgin Girlfriend Making Love With My Dad' - Boy seeks Help

Dear Readers, I am 23 years old. I have been in love with my girl who is 18 years old. We have been dating for 2 years now. She was caring and loving. She told she is a vírgin and we had agreed to have séx after marriage. We were only concentrating on our studies. She could come home at any time. I stay with only my dad and two little sisters. Early this week I had taken my little sisters at my auntie’s place. When i returned home, I found my girlfriend in bed with my dad. This was terrible and shocking. That very day my dad decided to chase me away from home. My GF never bothered to call me to say anything about it.But today she has called me crying that I should forgive her of what she did. Friends I am confused cos I truly loved her but I don’t know what to do. Should I forgive her or I move on? Apparently am staying with my auntie. Please advise me