It feels good because you are getting attention, in some form or another, from another human being. We all want that. We all crave that. So, of course it feels good. But she's using you. Even if she doesn't realize it, she's using you. She has a boyfriend already. Whether they are blissfully happy together and bound for eventual marriage, or desperately miserable and bound for eventual break-up... or even somewhere in between... it doesn't change the fact that she is presently with somebody else.
Most people need time to get over a relationship before they can seriously enter into another. You can't have that time to get over the relationship WHILE STILL IN IT.
Believe me, I am 100% not judging you here. I completely understand why you may have a hard time letting go. You thought maybe you saw something in her that is worthwhile. You thought maybe there could be something special between you two. It can be hard to look past that. Think of it this way, though... if there IS something there... if you two truly could be something real... well, then that would still be the case once she has dealt with her current relationship. Once she has broken up with him once and for all (if that ultimately is her decision). Once she has taken some time to heal, to grow, to get back up.
There are just way too many risks by getting involved with somebody in a situation like this. She could be using you as just a rebound (intentionally or perhaps without even realizing she's doing it). She could be somebody who just loves getting attention from guys and doesn't care about their feelings. Or, heck, there even could be a chance for you two given the right time and distance from her current situation... but rushing things could wind up ruining what could have been something great.
You ultimately have to do what feels right for you. We can't tell you what to do, you have to live your life. But, I just personally would never suggest continuing a situation like this. If you can honestly be 100% okay with currently just assuming nothing will ever happen between you two and thereby go back to just being friends, then great. Do that. You never know what may happen in time, but at least that way you aren't just waiting for her hoping she'll decide she wants to be with you. BUT, if being around her even as a friend would just make it too hard for you not to want more, than I think you'd be better off away from her for now. Which, again, could even change in the future if her situation changes. But, it would just be better for you not to get involved in this sort of thing. More often than not this kind of situation only leads to hurt. Not that it is 100% the only possible conclusion, it's just that so often a situation like this does not end well.
Good luck to you either way.