Look, you have to come to terms with the basic fact that, no matter how good you are at something, someone, somewhere is better than you at it. And, the off chance you're a one of a kind, there is someone out there coming for your title.
There is a dude out there with a bigger Johnson than you, a better lover than you, who fucks better than you. And, it's possible who your chick used to bang is all three.
But, none of that matters, at all. She isn't fucking him anymore, she's with you. This is her choice and she made it of her own free will and accord. So do your best to love her, and give it your all, and let the chips fall where they may. She's not with him anymore for a reason, and choose you for a reason. That's what matters.
You can't change the past, and this past isn't your cross to bear, it's hers.
Focus on what matters.