My father and our drifting relations

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I recently came to know that my father used to beat my mother and used to cheat on him in the initial days of their marriage.Now I came to know this and they are married for 17 years and they are now happy with each other and my mother has forgiven him, but after knowing this whenever I see his face or i am with him I feel extremely sad despite of knowing that she has forgiven him and now have no hatred or bad feelings towards him. I am feeling I am getting drifted apart from my father whom I used be closed to and tensions and arguments have started appearing between us.I am not able to concentrate on studies or something else. What should I do?