I disagree slightly with Poopy. I have been cheated on (as im sure the majority of people have). It doesnt, and never has, affected my relationships with new boyf's. I treat a new person on how they deserve to be treated, not based on someone elses actions.
Basically, if she thinks you are up to something when you have never given her cause to think this, then you are banging your head on a brick wall and the whole relationship will be a stuggle.
Remember, at the start is when things are good. Relationships get harder and harder as time goes on. If you are finding it difficult now, imagine 1yr, 2yrs, 3yrs down the line.
The first six months should be amazing, not hard work.