How to overcome stresses. Situation.

I m in relationship with a girl from 10-11 months and I was virgin before her she was my first time but I realised that she wasn't virgin she didn't actually bleed and I penetrated very easily.i had no problem and penetrated in first attempt .and I asked her many time about her past life but she always told that she hadn't involved in any relationship. I don't know if she is lying but this thing is hearting me too much .I m stressed from last month.i don't know what is wrong with me.i can't focus on anything when I force myself to go to study the whole scene comes to my mind . that she was with someone I cries.i feel cheated I answers her call and behave normal I don't know what to do in this situation.