I think it's very unrealistic to expect anyone to "let go" a feeling of bitterness and hatred about someone they feel, even if unfairly, was instrumental in ruining their life. This didn't happen once to her, a couple of years ago; it's happening every day. Every day, she faces the world as a lone parent, with the stigma and reduced standard of living that that involves. Every day, her child faces life as a child whose father doesn't live with him anymore, with all the well-documented disadvantages that entails. Like it or not, your uninvited entry into her life, has made it – and that of her child – very different (and possibly, at least temporarily, far worse) than it would have been without your presence. It is a leap of faith then, to imagine that she could just "let go" after a certain amount of time. People let go, move on, when they have reached some kind of equilibrium in their lives, and whether that means justice, or a new start, or some other means of emotional closure (to use that horrible psychobabble term) without that, it's almost impossible. That’s why figures like Medea, Dido and Cho-Cho-San exist, and in the case of the former two, have lasted for centuries – because they tap into a truth about the human psyche when faced with betrayal. They’re not comfortable in our world of “moving on”, “letting go” and offering your enemy a nice cup of tea; but boy do they make us confront the truth of our emotions and how forgiveness and getting over it don’t happen automatically or soon – they are something that will happen if she sees that you are a positive influence in her child’s life and she’s confident and happy enough in her own life, to not be threatened or feel a bit betrayed by that. But being confident and happy when the rock you built your future on, has turned out to be sand, is a tough call and may take years to achieve.
Sorry I’ve gone on a bit and I’m not saying this to attack you or make you feel bad, but just to put a viewpoint to you that if you bear in mind, will help you to contextualise some of the shit that may come your way in the next few years and adjust your expectations so that when things go norks up with her, you won't be too frustrated, angry or incredulous about it, you'll expect it and be able to deal with it.