I'm having trouble trusting my girlfriend, what can I do?

Plenty of more fish in the sky. there is a girl who is going through this same stuff u are experiencing, its time to drop her and move on. Yes it is hard as u have a past with her, but once u think she is cheating on you and u get that gut feeling, its a very bad sign.. listen to ur own instincts.. Dont let people tell you whether you should stay with her or not! if u feel it doesnt work, u no wat to do.. see if she changes, she might.. but doubtful.. goodluck man