Please listen to the other posters on this, not just me. Please consider getting a marriage counselor.
I guess my reasoning was, essentially what drives us apart is fuzzy feelings of something new.
What drives you apart is paying attention to other people at the expense of paying attention to the marriage. The fuzzy NRE feelings for a new crush? That might be the REASON behind the behavior. But the actual behavior of "not paying attention to my marriage" is the thing driving you apart.
I know that slices it fine, but it matters. What the actual BEHAVIOR is. You could have warm fuzzies for someone else and still manage your time so you spend time with your spouse. It's not about the fuzzies. It's about your time management.
You seem to believe sharing a GF will bring you together. Actually, what will bring you together is deciding to be spending time together. Again... time management.
Way back when this all started and you both cheated? Check it out:
Myself and my wife have been together for 13 years, married for 8 and have been strictly mono. A few years ago we both "lightly cheated" right about the same time (no actual sex, supposedly, but it did get physical) but if not found out/caught, probably would have eventually got to sex. I have realized we both must have been missing something for it to be timed so perfect. We decided to try and work it out and continued our marriage. Since then, to sum it up, we have both engaged in flirtatious conversations and maybe even some "more than just a friend emotions" with others which eventually "again" had means to get deeper if not found out.
You know something was missing in the marriage.
You both decide to fix it.
But then go putting the focus on spending time with other people instead of spending time tending the marriage.
You already know that did not work. Could not repeat that.
Could try something you have not done yet -- see a counselor to HELP you make the repair plan this time. When you guys made the repair plan before, the plan did not serve your needs well. It does not seem you made a plan. It sounds like you agreed it needed to be fixed, but you didn't know HOW to go about actually fixing. You both could get help with that this time.
Could try something you have not done yet -- date each other, pay attention to the marriage. Rather than dating other people and paying attention to them. Figure out what was missing in the first place. I wonder if you both got bored with your roles.
You mention a 3 year old -- did you guys get run down with baby blues and in tending to baby, forget to tend to each other? Forget to get a baysitter to make times to be relating as an ADULT couple? Rather than only relating to each other in mommy-daddy parent mode?
Then the new people provided that kind of attention -- "See me as more than a parent! See me as an adult person!" and you both were starved for that kind of attention?
Could that be some of it?