@helphelphelp123,
Yep, you screwed up, royally.
It was one thing to I assume kiss another woman and maybe more.
It was another thing to out and out lie when you were confronted about this. It also didn't help that this happened more than once. You've used up your '
Get out of Jail Free' cards here.
I think you're right, that anything in this situation will screw up your relationship with your girlfriend, so why not be an adult about it and let her go?
And then consider, perhaps, counseling, to figure out why you would put yourself into such a situation. Your ex has you by the short hairs and you allowed that to happen. Frankly, you don't sound over her. If you're not, then you owe it to your girlfriend to give her the chance to find someone who's not hung up on someone else.
And also consider what it means when your ex does this. If it happens again, she will do it again. You've allowed yourself to become a victim. Try not to put yourself into that kind of a bind again.