Facebook is the number one way ppl can cheat easier at this day and age but also it's the number one way to catch a cheater as well. Damn social media! Anyways, I can tell u that your gf has a very high chance of cheating again. Sure she may miss u and feel guilty after the fact... But once she gets comfortable with u again, she may retreat back to her old ways. If u keep on forgiving her and taking her back, she will just know you r a pushover. Yes u want security and to eventually grow old and have a family but u will one day thank god u found out before doing something stupid like marry her and have kids. U have invested lots of time on this relationship and memories but u can have all that with someone else u will b able to trust!