Oh HM, you'll do yourself a lot of good and do DD less harm if you can change your internal narrative a bit.
Some woman from the 1980s did not cause the end of your M. The problems with the M were internal to the M. And, whoever she is, his GF or just-friend or whatever, she is NOT NOW an OW. You are D. There is no 3-way R. Your EX-H's LL is none of your business, and unless you are the third wheel in it, then from your position there is no R that you have with XH in which you are even in a position for someone to count as OW. And, the woman from the 1980s is not taking your place at EX-H's family's table, because the fact is that you don't have a place there now at Thanksgiving. They are HIS family.
Sorry the holidays are hard, but for your own sake try to move away from feeling like a victim and feeling like you remain entitled to things that you thought you would have in your life.
As far as DD, I've no idea what your custody agreement is, but stick to it. If Thanksgiving is one of his days, then at most all you should do is ask nicely for an exception to pick up DD early. No surprises, no drama, and even better, only nice, business-like communication through email only. Maybe if DD feels it is OK with you for her to enjoy the whole day with her father, then she won't have to disrupt her holiday and be jerked around by the D-stuff in the middle of the day. But, she won't feel that way as long as you drag the old emotional baggage/victimhood/old entitlement stuff around with you.
Set both you and DD free by making a fresh start as an independent empowered woman who does not let past hurts dictate the shape of her present and future as one of the things you are most thankful for this year.
