Why can't I let go of the past? I can't seem to overcome it and I'm tormented by it?

My mom grew up in a way where verbal abuse was normal, she doesn't understand that it's wrong and how it affected me. She says she's sorry for 'whatever she's done or currently doing to to hurt me' but I still feel a grudge. It's as if I need her to understand HOW she hurt me, why can't I just accept that she's sorry even if she doesn't know the reason? I feel resentful even though she's sacrificed so much for me. I wanna forgive, move on but my mind just won't let me, I don't know why it's so stuck in the past. Do I need to see a psychiatrist or something?
Why can't I let go of the past? I can't seem to overcome it and I'm tormented by it?
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