I have a major problem or something please help?

I am in a relationship have never cheated but I am feeling something for someone else. this person does not have a good marriage and everytime I get a whiff of a bit of trouble in their marriage It excites me and I want to try to weasel in with him. I know this is terrible but I have never felt this way about anyone. What am i agoing to do? This has been going on for months.