How do I reconcile my feelings about my X and my current g/f?

I find myself in a difficult relationship situation I’ve never faced before. I have been dating an amazing girl for several years now. We live together, and though we have our ups and downs (and sometimes very big downs), it is a very rewarding and stable relationship all together. She is very easily a girl I could see myself marrying, and I’ve even contemplating that idea at great length a few times (though it was ruined each time by one of those low lows).

But then comes my X g/f. She was my immediate X prior to this current g/f. We dated for several years as well. It was a much more dramatic relationship, the highs were much higher, and the lows were much lower. But there was a level of passion in that relationship that I’ve never quite matched with my current beau. Several times since we broke up, and even during my current relationship… my X and I have nearly gotten back together. Typically one of us is much more into the idea (switching off each time which of us it is), so it fails to amount to anything. But even now, after dating my new g/f for a few years and having been long split from the X…. I find myself at an impasse to eliminate my feelings for her. I’ll make it for weeks or months without even having a second thought about her, but then I’ll have sudden pangs of memories and an almost need to see her. I get that knot in my stomach and chest… it’s very difficult, sometimes far worse than others.

My X left me for another man, and it didn’t work out for distinctive reasons I could’ve told you from the start (he was more of an escape from her problems with me). She’s since moved onto another guy (after a half-serious attempt at woo’ing me back), but it also seems to be a relationship bound for failure. In the mean time I am trying to deal with these feelings when they arise, and figure out what is going on in my head. I have this amazing girl right now, and I truly enjoy my time with her and love her… yet I can’t shake these feelings I get with my X g/f every other month or so.

I wonder if maybe my subconscious is just using my X as a scape goat, because maybe I’m not as happy as I think I am in my current relationship? I wonder if maybe I really do just love my X and I’m trying to bury that with my new relationship? I wonder if this is some chic flick and I’m in love with both of them (oh what I’d give for a society that accepted multiple women with one man..)

How do I reconcile this?