What are my options, i'm a bad person?

My husband and I have been married for 4 years, I used to really find him attractive,and we have a baby girl together. I am really beautiful (don't mean to sound vain, but i can't deny it) and loads of men proposition me all the time I have always shaken them off but I started sleeping with my gym instructor for a fortnight now and I have become so involved in him that I think about him all the time and I feel that I'm cheating on him when I sleep with my husband. I can't control my feelings and I'm not usually so selfish. i want the best for my girl and my husband is a brilliant, kind and supportive man but I DON"T find him attractive anymore. WHAT DO I DO? Please help me