What should i do please help me ?

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Hi i am in very weired situation .My gf caught me getting intimidate with her friend .It all goes like this.... I have been in a relation for around 6 months , her friend helped me a lot to to know abut her and later i proposed her to but since like last 2 monts she was geting a bit annoyed by me {i dont know why}i guess some family problems.Few days back i went to stay at her place{her parents were out}, in evening she called her friend to come there which i didnt know ,She was lying in her bedroom with lights off, when i came i just started kissing her and got in fully comprising position {and to my surprise she didnt reacted as she was somone else}, and inthe meen time my gf came into the room and she noticed as doing it all.She was really pissed at both of us , i being the most .My gf left the room and her friend confesed that she had a crush on me and she always fantisied about me .I have explained all to my gf but she still in no mood to be on talking terms to me.1 big mistake had reuined my very beautiful relation ship please help me what should i do in oreder to resolve this problem please rely. thanks.