You were being too nice to her, so you fell into the category of "NICE GUY" as opposed to "A GENTLEMAN". You did not maintain your dignity or your place in the relationship by constantly giving and not expecting anything in return. Thats when she stopped caring enough about you, she started taking you for granted and eventually slept with someone else.

Guys: Dont be nice, be gentlemen. Please stand up for yourself in relationship, be confident, treat your women well and expect, err demand the same in return, its your right. If she doesn't return the favor, dump her and find someone who will. Only way to succeed in relationships and avoiding becoming doormats.