Damon hadn't necessarily wished that his brother would be hurt by Elena. So determined that he didn't deserve her, even when he could have withheld information from her and make Stefan look bad and himself better, he never did. Not only because of his belief he didn't deserve her, but because he had wanted her to make an informed decision, knowing all of the facts.
And that is what she had done, and him being undeserving of her didn't really come into it anymore, because he knew that she loved him, and he loved her, and that even if he didn't deserve her she thought that he did. And that was all that mattered, at the end of the day.
Damon hoped that her and Stefan could remain friends too, despite how difficult and awkward it might be at first. They needed each other. And Damon had to trust that what he had with Elena was strong enough that Stefan wouldn't come between them.
It was a difficult thing for him to do, but something that was completely necessary for him to do, also. Elena loved him. Wanted him. She wouldn't do anything with Stefan behind his back, he was sure of that.
He nodded his head and smiled. "I'm happy too, Elena. For the first time in my life, I feel truly happy."
"And we will be okay. I'm certain of that. We always survive. Always come out a bit stronger on the other side," he said.