Is it normal to have a hard time forgiving myself even when the person already forgave?

I'm already friends with the girl I gave a hard time for 3 whole years; years she'll never get back. After how I treated her, my other friends said I'm lucky she was forgiving.

It's that she killed me with kindness and I felt in debt since that day. Once in a while, I reflect back and feel shame at how my former self was; judgmental, rotten instigator. Her b-day is on April 12. Is it normal to feel shame at times and have a hard time forgiving myself?

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Then whenever the topic of school bullies comes in the news or in a movie, I'm back to feel so ashamed, disgusted with my former self.
Is it normal to have a hard time forgiving myself even when the person already forgave?
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