Sorry you're dealing with this. It seems to happen quite a bit....but I think it's because "new" relationships are an unknown...and they gravitate back to the old "familiar" because it doesn't require the time to get to know each other.. and they think they can fix it.
If someone tells me they are fresh out of a relationship, I lay low for quite awhile to see if they are predisposed to this behavior. (*hint* most of the time they aren't...especially if they are the ones that got dumped)
When a “real” relationship breaks up, I think the next relationship will be a “disposable” relationship. It’s needed to get the old taste off of your pallet.
Sad but true. NEVER be the NEXT one after a BIG one. You’re in for heartache.