I see a few things here....MWM.
I guess my first question would be what did your Mother say/think you did to wrong her? The fact that she had a brain tumor and surgery, but remembered to tell EVERYONE NOT to tell you tells ME that she thought about YOU in a way that maybe consumed HER day. The fact that she left you out of the will could have been an act that was overlooked by her as she became more ill - wills are drawn up a lot of times when people are scared or believe they are going to die. So it is a possibility that she had the will drawn up before she remembered to add you. (it's possible, not likely from what you wrote, but nonetheless possible)
I think YOU are missing the actual lesson to be learned here. You already HAVE forgiven her - you just never got a chance to see her forgive YOU. THAT's the real issue to me. And the fact that she has passed on, means that you will never GET the opportunity to hear HER say "I forgive you." And how sad for HER that she had to walk around with such a grudge under her arm like a knot. It must have been very uncomfortable and time consuming to hate someone so much. When you hate someone they own you.
The fact that you are going to a therapist and working on this is the best thing you can do. Trying to speed read books or learn how I would forgive maybe a wonderful addition to your therapy. Several people here have suggested some wonderful reading. Trying to make the pain go away quicker isn't going to help youin the long run either. It's there, it's real, it is present in your life, it makes you made, it's consuming you and you obviously want it gone - but what stops you from making it go away? No one here can answer that - you can ---eventually with a good therapist you trust and feel is working with you to make you become the best you there is.
I'm sorry here for your Mom. She missed the life of a wonderful daughter who like anyone in the world had faults. She missed the life of 2 grandchildren. She missed the opportunity she had within her to say I forgive you, and get on with her life. Instead she was forever stuck on remembering you right up till the end. That's tragic when anyone consumes their life with another person so much they exclude them instead of forgive them.
I don't have all the answers on forgiving anyone. I think we all do the best we can to be kind and when someone trespasses against us, we pray for our own transgressions against others as well.
the mere fact that YOU are in therapy about this says to me, that YOU are willing and maybe have forgiven your Mom, it's just that you're lacking the sound of her words saying I forgive you.
It's just a guess. Personally? When I let go of anger it's been replaced with love, the more love I have, the more I feel, the more I feel the more I'm able to give, and the easier it becomes to forgive even the largest grievance - for me it was my x molesting my son and selling him for crack cocaine. The more I pray for the ability to forgive him, and do it...I found there was happier things in my life. He'll never be near us to say "I'm sorry" - he's a psychopath, he has no conscience. So I had to take what I could get for ME and deal with it. Sorta a backwards forgiving.
Hope this helps....
Hugs
Star
p.s. IF it's any consolation I read your words, and I think you are a wonderful friend, Mother and given the chance daughter. Remember....your HIS daughter if your no one elses.