Katherine doesn't often regret her actions. She does what she wants, does what is necessary; and she never apologizes for the destruction she causes. It's only when she considers her relationship with Damon that she wonders if maybe she should have done something different.
Whatever it was she said to him in 1864, whatever it was she did to him, it made him continue loving her for almost a century and a half. There is a devotion in his actions that she knows she doesn't deserve, and she cannot help but prove herself right.
She tells him she never loved him, says it was always Stefan. She says she came back for Stefan, maintains the lie that she'll kill him herself if it means she can have Stefan.
The way he looks at her now hurts her more than she'll ever admit, but it is necessary. He hates her now, although she doesn't blame him; and the anger in his eyes tugs at her heart in the worst possible way.
She feels a sense of relief when she realizes she has managed to crush whatever love he once held for her.
Katherine may regret the way she has played Damon like a fiddle, and the first time was more of a mistake than anything, but the second time was necessary.
Katherine Pierce does not deserve to be loved in the way Damon is capable of. She does not deserve that sort of heaven.