Cheated and stayed?

I am asking because this has happened to me. I've been with my SO for 7 years. I've heard of the 7 year itch but didn't think it to be true up until this happened. We were in a very rough spot in our relationship and he left me. He started spending time with his best friends older sister who completely took advantage of the truly selfless person he is. She got pregnant after a month (of them seeing each other and us being apart). A mutual friend called me after about 7 months telling me how miserable my SO was. The girl had quit her job and already had 3 children. My heart broke for him having all that stress and weight on his shoulders. I truly believed he had made a mistake. I called him up after 7 months and as soon as he heard my voice he broke down sobbing. We have been together ever since and are now married. I am slowly forgiving him and he has gone above and beyond to show his love for me since that day. He has been willing to do anything I ask of him to help gain my trust. I'm just curious how do you get over constantly thinking that it's YOU that is the reason he cheated? The other woman is totally opposite of me. And my husband has said and done everything he can to reassure me that it was a huge mistake on his part and nothing about her is better then me. Anyone experience anything similar? I also have so much guilt associated with this because now she is not allowing him to be a part of his child's life (legally she's still married so the child is legally her husbands)