There's a girl in my friend group over here, we both liked each other but one of us always had a partner/ we then went to uni. I'm now very happy with my girlfriend, but I know that feeling of wondering what it would be like and always thinking what if we had given it a go.
Although, I'm sure my girlfriend> her. So unlike Hamish it isn't that bad. As for advice, well you're both in relationships so breaking them both seems a bit far if you're still happy in your current relationship I think.