Well, I think there's a lot you have to consider.
First thing is first, however: did you make a promise to the man you're currently with? Are you exclusive? If you are, maybe you need to sit down with him and explain to him that you're feelings are not truly committed to him (Honestly, they aren't and you know it...).
Secondly, I think it's quite common to still have feelings for old flings. I still have some emotional "feel goods" about people I dated in the past - it's completely normal. But there was a reason you split up, there was something that you felt made the person incompatible for a life partner. Sometimes situations change, but usually the emotions do not.
I have a great friend that I absolutely adored. I'll never forget her, she was the best thing to happen to me at a certain time in my life. Time went on, however, and she went her way and I went mine. Eventually I met my wife, who I have a profound relationship with. Did I love the other woman? Of course I did, but it didn't develop into anything that I now have with my wife...
and if my wife ever asks? I lie my *** off and tell her nothing was ever there...because I want to stay married to my wife :P