I agree, it is very hard. I had a therapist once that had me write a letter - that I would never send. You address it to the person who hurt you (your mom), explain how you remember feeling about you n her before she hurt you in detail, then describe all the ways she hurt you in detail (not just the actions n words she spoke - but any thoughts n feeling you had during the harm as well), then describe in detail how it still affects you, then - explain if you were to see her now what you would need in order to "feel good again"

Then end with "I forgive you"

After you have written it all ... Save it n re-read it til it no longer hurts. You don't have to (n should not) do it all at once. Do it a little at a time. When it starts hurting too much - put it away awhile.