When I was much younger (17 years ago), I knew my GF cheated on me, then promised not to do it again (she confessed on MDMA).

The thing was, she was cheating later on me with basically everyone she could find, I had my suspicions, but kind of chose to be blind to it (she even hinted it quite a few times).

Then there was the phase where she asked to be in open relationship (which I know nowadays it means she was cheating and wants a reason to validate it) and later asked me to dump her so that she could trade me for my best friend. I remember asking "you found somebody else, didn't you?" and she said yes in that not-sure-if-joking-or-true tone.

This was confirmed by a friend of hers, who was unhappy by her boyfriend cheating, so she spilled the truth.

I hacked that guy's account (ICQ back then), read their messages, to be sure.

So I lost a best friend and GF at the same time.

The most fucked up part is that the GF's mother teached her to sleep around, because the mother was in a bad marriage.