Ladies I need some good advice.

My boyfriend of 2 and a half yeas told me what happens between him and his Ex they were dating for 8 years. He told me he cheated on her with one of his classmates from college her and him were close friends and he told me he cheated for a whole month before he told his ex and moved out of their house. He didn't end up dating that girl because she moved away. It just creeps me put because he is such a great guy and I would never think he would be capable of doing that he told me he would never do it again because it tore him up inside and he knew it was wrong the only reason he did it because to get out of his long relationship. Now I feel like he's a cheater like he would do it to me I always belived once a cheater always a cheater?