Anger is a secondary emotion.

Maybe you're viewing what happened as if it requires more than thought to let go of it. Everyone has different processes to deal with grief. But in the end through therapy the solution would take time to get to that root emotion. And once you do you'll experience that feeling and be able to mentally let it go once you understand why she did what she did and why you still feel the way you do or why you're still allowing the past disturb your current natural state. So simply put by digging deeper in your root emotions and digging into why she treated you like that which will probably lead you to analyzing her parents or past relationships that led her to having the same secondary emotion she chose to manifest in you as well.

I can personally say I fully let go of the 20+ yrs of manipulation and mental abuse I received from her by realizing why she was that way. When I came to the conclusion of how she formed these habits and practices I understood her and my grandmother and grandfather a lot better. I even bounced these ideas around amongst my siblings to get feedback and comfirmstion on things I didn't see. In the process I helped one brother but the other is still holding onto the anger. Thinking that she somehow still has control of who he currently is.
By learning these things it allowed me to notice the things I picked up and allowed me to let go of them and the deeper emotions that I thought made up my identity.