I have had to forgive a few people in my life too. I wonder if the way we forgive people might be hardwired into our makeup at birth. Some people seem to be able to forgive easily; for others it is almost impossible.
For me, forgiving somebody is almost a selfish act. MY LIFE is TOO SHORT to spend it worrying about, thinking about, or trying to get even with SOMEBODY ELSE. I don't think forgiving somebody means that now everything is OK and they were right to do whatever they did but it does mean that you accept what you cannot change and get on with your life. I don't want to give somebody who wronged me the power to continue to mess up my life.
Obviously, it is harder to forgive a mother than to forgive a passing stranger or even a spouse or a child. The mother/daughter connection is one of the deepest and most complicated bonds that exists and when that goes kaput, it is bound to leave you with a lot of hard-to-deal-with feelings. However, it is still true that YOUR LIFE is TOO SHORT to spend it being upsset by something you can do NOTHING ABOUT.
I know that just saying GET OVER IT is not helpful. It sounds like this might be a good time to seek some counseling on this issue.