Had s** with my god-sons mom

I had a classmate who was 16 and had a son He went to Canada to study after a year. We were in Hong Kong. He told me to look over his son from time to time since he was also a good friend of mine. I didn't have much time during the day due to school, so I usually go to her place on friday nights. She tells me about her feelings and what problems she has. I try to solve her problems. One night, she was depressed and called me to go to her place. I did because I was the only one who knows about his son. We had a long time and she was drunk and kissed me. We had s** afterwards. For the next couple months we had s** 2-3 days a week. I knew this was wrong but I knew I needed to look after the son. Occasionally, I skype with my classmate and tell him about the conditions about the mom but I never mentioned about having s** with her. We stopped having s** when I started to date another girl and She also dated someone else. Now, I am in Canada and I skype my classmate on a daily since theres not much to do in Canada. I feel guilty everytime I talk to him.