As a girl dating a guy with the same problem (actually, I think you last longer than him), I feel like I should comment. First of all, someone who would dump a guy over something like that without any other reason has their priorities all screwed up. Second, you didn't fail as a man, she failed as a PERSON. Cheating on someone and dumping them for the other person is low down, but to do it when you're vulnerable because of how this issue was making you feel is just cruel. This is not a reflection on you in any way, this is a reflection on HER.

You ask what the point is in finding another girl? Because not all women are so shallow. I would never cheat on my man, even if the offer was made by some super stud that could stay up forever and knew all the tricks in the book etc. What will we do instead? We will go to doctor(s) and a sex therapist to fix these issues or find a way around them. This is what kind, caring, non-shallow people do. They don't just abandon you because things become difficult or disappointing.

My whole point is that this isn't your fault, so don't blame yourself. And this doesn't make you less of a man.