What You Can't Forget
You're being silly about this whole thing, and you know it. She was only supposed to be a bit of fun; you knew from the start that she was crazy about someone else. It's your own fault if she turned into something more. He's here, trying his very best to please you, and she's off with her own "him." And if you think she's thinking of you, you're dead wrong.
You mean nothing to her, and you mean everything to him. So when he caresses you gently, try not to remember the way she shoves you against the wall, the way her wild hands roam all over your body. When he kisses you softly, don't think about how her tongue attacks your mouth or the feel of her teeth clenching your earlobes. He's no match for her, and you know it. No one is. It's not his fault.
Of course having him inside you can't compare to her fingers fucking you roughly. And his soft grunts have no chance of turning you on the way her breathy moans do. How could they?
Think about this rationally, now. You're in love with her. That's fine: everybody's in love with her. But she's not in love with you - so just try to forget about her, at least while he's here.