My girl betrayed me once...and I can't seem to get over it...?

i have a problem ...to be honest it is MYSELF. My girl betrayed me once and came back into my life. She was away from me for 3 months and the reason was another guy. I knew this and I had a very very bad time then. Donno what happened, but she came back into my life again crying. She never told me why, all she said was she missed me aswell and It was upto me to accept her or not. I accepted her. It has been 9 months together again. There have been good and bad times. My problem is, I don't really believe her on anything she tells me until it is conclusive for me. There are times when I try and cross check and there are times I leave it alone. But my first reaction is negetive. I start thinkin what could be the motive behind and stuff? Is she trying to fox me into something etc. & my behaviour that follows troubles her more often than not and we end up fighting. We neither are happy together most times nor will be happy apart. The negetive experience