If you tell her it's b/c you feel betrayed then it IS about your hurt feelings and punishing her for the betrayal.
Is that a horrible thing to do? I don't know and can't judge you for it b/c I know I'd feel the same. But either say it's the book or your schedule or the money or the time away or that you don't want to be there nor do you want them to be gone that long at this point in time ETC b/c if you tell her the truth, it'll be used against you emotionally if not otherwise and it will NOT play out the way you think. IT's going to look as if you could not handle your h's "love for OW" and refused to accept it and are "punishing those who DO accept" his new life and blah blah blah and I don't think you want to look that way.
Did the MIL promise you not to meet OW or what? Regardless, you may at some point say your R with her "has obviously suffered" b/c you didn't feel "THAT THE M OR FAMILY was supported b/c instead, she chose her son's new interest and terrible fathering, (ironic that she plays the World's best grand ma routine but keeping the parents married was not a priority???) over standing firm and telling him like it is. Although do you KNOW that she never spoke up?
I ask b/c as I said here, I'd sure as hell tell MY SON (22 right now) what I thought if he pulled this stuff BUT I don't know what I'd tell my dil. MAybe that convo with son would remain private...don't know but I am sure he had to justify the A to her "Not happy for YEARS" and "W does not meet my needs" and blah blah blah. BTW, what was his parent's M like?
Just something to consider when you tell her "NO" again. Just consider NOT sharing ALL the reasons you are not up for it this summer."Maybe later....like when you heal"....(or when the kids turn 18)
FYI you really are doing well. Look at some old old posts if you can stomach it and see the progress you have made...it's a lot.
(( j ))
M: 57 H: 60 M: 35 yrs S30,D28,D19 H off to Alaska 2006 Recon 7/07- 8/08 *2016* X = "ALASKA 2.0" GROUND HOG DAY I File D 10/16 OW DIV 2/26/2018 X marries OW 5/2016