Donna--wow, do I know how you feel. it's as if things aren't bad enough, now there's the OW trying to be another mother to our kids, and we're supposed to just accept it and go on.
In my sitch, I found out in July that my D13 had been spending time with OW and her family since January. She never told me about it because she didn't want to hurt my feelings--or have me blow up like I did when I found out they all had dinner together without any preparation for D13 at all. I was crushed; however, a couple of months later D13 had a 3-hour-long chat with me, which she initiated. She has known all along that OW broke up the marriage; she figured it out on her own. It was fairly obvious anyway. She feels very much like a low priority to her father, sort of like his 4th child instead of his only one. He has tried to force a relationship with OW and her daughters, and D13 is pushing back--which is very out of character. She doesn't like them at all, wants nothing to do with them, really doesn't want much to do with her father at this point, and predicts "she'll dump him once she's done using him" to help her muck out horse stalls and watch her grandson. In short, D13 "gets it" about her father's character, about OW, and appreciates being able to talk with me. There may come a day when I'll have to face OW--most likely at 8th grade graduation in May--but there will always be an opposite end of the room to sit.
And listen--no, your xH was NOT right, you are NOT defective. It's a natural part of being left for someone else to feel discarded and rejected. But the flaw is not in you. Sure, learn what you can about whatever your role might be so that the same issues don't pop up in future relationships. But it was xH who fooled around with the neighbor and didn't honor his vows or his family. And THAT is a fatal flaw.
M60 H52 D20 M14 yrs OW-old gf from 1986 bomb-5/18/08 H filed for D-9/10/08 D final 4/24/09 xH remarried (not OW) 2012