While some men have egos the size of Texas other men suffer from the occasional bout of self doubt. It's nothing to be ashamed of but while some men are thinking their cheating wives and girlfriends are evil incarnate you may use this weak moment in your mental makeup to wonder if you might, by some wacky mistake of the cosmos be the reason for her discontent.
Hey! It's an incredibly enlightened thing to consider but you can breathe again any minute now because that is almost certainly not the case. You see, she loves you a lot. She probably loves you more than you realize. She doesn't love just anyone. She loves you. She might even think you're the greatest thing since peanut butter and jelly sandwiches were created or something just as wonderful.
She's not about ready to jump on over to some other man because you're not good enough. She's just managed to get you properly trained in the fine art of letting the seat down.
You see women generally cheat on their men not because he isn't good enough but because there is some need she has that isn't being met. It's not about the eyes or what she can see of you. It isn't about how much money the other guy is making (remember she's staying with you). It's about some deep need she has that isn't being taken care of at home.
That still doesn't mean you aren't good enough for her unless you are a very talented, dare I say, gifted mind reader in your chosen profession. Unless of course when you were just getting started as a couple she told you "oh yeah, there's this one thing I just can't live without. If I don't get it; I'll probably cheat on you. And that thing is (whatever the heck that thing is)." My money is on something along the lines of "that never happened" which kind of leaves you off the hook.
Now the only thing that remains is for you to figure out what she got from the other guy and how you can give it to her instead. The other thing you need to do is get a handle on your own personal insecurities. It's one thing to believe you are the luckiest man on earth to have someone who loves you by your side.
It's something else entirely to go through the relationship waiting for it to all fall apart because you are somehow unworthy of her. That's how good relationships go bad fast and is a complete and total recipe for relationship disaster and discord. You must understand your value to the relationship in order to make the relationship work for both of you.