You have a reason for these thoughts and its called your past relationships. Just don't take it out on her- doing so will only push her into doing it. I have been with my husband 11 years and I still think he cheats even tho he never has- its all in our heads! I never tell him I feel this way cuz deep down I know it's my own issues. Make sure u trust this girl until she gives you a reason not to- trust is everything in a relationship.