I was 8 months without seeing my GF some years ago. I didn't cheat. I had opportunity. I have had girls take off their clothes in front of me before. I didn't cheat.
It was not because I was bored, or because I was horny.
So why did you put yourself in a position to be spending time with another woman? Not only spending time, but enough of it that you developed feelings and a relationship?
I don't claim to be a master of relationships. I've made lots of mistakes. But cheating isn't one of them, nor will it ever be.
And being a Partner 101 means not secretly hanging out with someone else, even if its strictly platonic.