Just block this guy from calling you so he can't keep getting back into your life. Things seem incredible with him because you weren't together long enough to get past the fantasy stage of the relationship. If he came back, you'd most likely find that you don't want to be with him. If your partner of 12 years has the things you want, then don't throw all of that for a guy who likely won't last a year. Just stop taking his calls and messages.
“This planet has - or rather had - a problem, which was this: most of the people living on it were unhappy for pretty much of the time. Many solutions were suggested for this problem, but most of these were largely concerned with the movement of small green pieces of paper, which was odd because on the whole it wasn't the small green pieces of paper that were unhappy.” ― Douglas Adams, The Hitchhiker's Guide to the Galaxy