@sunnyjim,
I understand that you're having your cake, eating it, too, and are feeling a bit guilty about it. And so you should.
Sorry, but I don't really have a boatload of sympathy. You are well aware that this is a problem.
Here's an idea. Break it off with both of them.
Crazy, huh? Hear me out. You are cheating on your long-term. This is no good and, frankly, it smacks of boredom. Can it be salvaged? Maybe, but you've already strayed. It does not mean you will continue to do so but you have already gone to that well. Unless you want to work on that relationship - as in counseling, the whole nine yards, I believe that you've already taken a shotgun to that relationship's head. You can revive it, but it means real work.
As for the chippie on the side, the reality is that many affairs are driven by the excitement of being naughty, and the fear of being caught. Remove those and you are often left with a rather mundane scenario. And recognize, also, that you don't see the chippie at her worst. I doubt that you see her with curlers in her hair, when she's trying to figure out the tax forms or is cleaning the tub. It's an unfair comparison to your current girlfriend, when you get right down to it. Switch them in your mind for a moment, and see the chippie as she negotiates bad traffic, has a fight with her mother or hasn't put her makeup on yet - all of the thousands of things that are unattractive that you see your current girlfriend do/say. Plus building a new relationship on the foundation of an older breakup/affair is truly not a great foundation at all. And since your chippie has already proven that she will be complicit in cheating, will she do it to you when she gets bored? You're not the only one with a premium on boredom.
I do recommend counseling even if you don't want to salvage your current relationship. You might want to explore why you're doing this, and find some techniques for keeping a long-term love alive and combatting boredom. It is a two-way street. Some of this boredom (and I have little doubt that that is what's going on here) is
your responsibility. Own up to it, and work on it.