if you strip away all the "addiction" related bits you still have an ex-gf who took up with someone else. that's the bare bones of it.
break ups happen. one person can THINK everything is FINE, while the other has secretly been clawing at the wallboard to get out. we see what we WANT to see, in circumstances and in our Other.
bottom line, she left and moved on. i guess one might want to ask themselves why they would wait around and see if the New flames out and THEN the ex comes crawling back to us, as an afterthought. i should think that would be more of a kick to the ole self esteem than the original departure.