People are people. Lying, cheating, stealing are part of human nature. Get used to it.
If you don't experience the worst, you'll never realise when you have the best.
I've never had a GF cheat on me (that I know about). But then I don't change my girlfriends as often as I change my underwear, like most young-uns do today.
My previous girlfriend was for 3yrs & 8 months. Then an 18 month break, and now I've been going out with my SigOt for over 2yrs.