All of these answers are so good. I know this contribution is so minor.
But my experience has been that the more than someone has wronged or harmed me, the greater and deeper my compassion for that person has run when I really consider the situation. And I'm talking serious harm.
No matter what s/he has done to you, his/her own suffering for the harmful or horrific actions will be far more than they would imagine, I think. For that reason, I generate sorrow and compassion for him/her with some ease these days. Simply because I know what it is to suffer.
It's not easy to do, I think. But if it's felt in the heart and mind, it's genuine and true. That's good for both you and the difficult person. The more you practice this the easier it is to do. And eventually it's like it's a part of you, that your heart goes out to those who act badly. It doesn't mean that you act as a punching bag for others or some sort of doormat, but you can feel the empathy needed to tolerate their actions and generate metta and compassion.
I hope that helps sattva. But please, more than anything please take these wise advices from others here under consideration. I'm merely pointing out a direction for the heart, that's all. And as I said, it takes practice and a lot of repetition at generating empathy
. Not sympathy, but empathy. To imagine the suffering s/he is creating with such actions and temporarily walk in those shoes.