I was actually amused when I read it. It sounds to me like she tells herself some flowery lies so she can believe he loved her so much he couldn't bear to hurt her anymore. If he really loved her, he wouldn't have let it go on as long as it did.
People tell themselves whatever they have to so they can deal. If she wants to believe what she had was love, let her believe it. But love doesn't relish in the vindictive the way she did and love doesn't end up alone and in pain. And no matter why she tells herself he left, she has no idea what the real reason is. She only knows what he told her and she ought to know better than anyone how well he lies.
Rather than leave her to avoid hurting her, it's more likely that he figured out that she was getting pleasure out of hurting the only innocent person in the situation, his wife. People rarely "love" someone like that for long. And knowing that's who she was probably made the justifications for cheating that he was giving himself a lot harder to swallow.
But in any case, in that whole situation, there were two dirt bags and his wife wasn't one of them. She proved that just by writing the "letter".