What is it she liked about the other guy? Find that out if you want to, but you will need a rock solid foundation where you will not lose confidence over it.

We all know the thrill of the chase and something new can be 99% of the allure. You hear it a lot "I don't know, it was just exciting I can' explain it"

It may not be you that is the problem, just life in general beats on you, and if you feel happier talking to someone else it could just be escapism.

Either way you will need to be rock solid so you can move on without it affecting you mentally, everything like this that happens ends up being a good thing for both people if the right mindset is applied