If an Aries man has trouble forgiving, it could be that he has the Moon or Mars in fixed signs (especially Leo or Scorpio). Every man is complex, and his Sun expresses only one (albeit important) part of his personality.
Many Aries men have Mercury and/or Venus in Pisces, the most forgiving of signs. He might be a warrior-martyr, initially feeling irritated and then believing he deserved to be a victim of your errant behavior. Then you can forgive him!
Comment below: Have you had difficulty being forgiven by an Aries man?