Had sex with someone that isn't single

I've been hooking up with this guy on a regular basis. It started out just being friends but he would always flirt with me and text me etc. After me and my latest boyfriend broke up, he wanted to hang out and I guess I was okay with it because I didn't think anything would happen and we had been friends for a few months but that night we ended up hooking up. I knew he had a girlfriend but it just kinda happened. I got very very upset with myself the next day and swore I would never do it again but he said he wanted to hang out again 2 weeks later and I caved and did. I've slept with him two other times and I know it's wrong but it's impossible for me to not. He's very attractive and tall and I don't have faith in men anymore anyways so I guess I like that I am getting sex without commitment. Am I an awful person? What do you think I should do?