This actually happened in my first M (except we had no children). I was a WAW, a fact I'm not proud of, but my exH was remarried within the year to a woman I had introduced him to.
The initial feeling was just a "funny" feeling. Yeah, he's getting on with his life...
Later it helped with my feelings of guilt. I was glad he was happy because something good came out of this.
MANY years later-- 25+-- I had a major meltdown one day when I realized he had gone on without me and had a life, while I felt I had been circling the tower for years.
BTW the two of them are still married ~32 years later.
A counseling prof I had at the time said that the person who leaves (or initiates the breakup) usually struggles with guilt, and the other one usually struggles with self-esteem.
Much of this depends on whether you parted in real rancor or just sort of "moved on." IOW how attached are you still to the ex. I'm sure children (especially YOUNG children) would vastly complicate the sitch.
