My Partner Hates My Adult Children

I really feel for you and especially at Christmas having to deal with this.
As Denbylover said, without a massive turnaround from your partner, there doesn't seem much else that you can do.
Sadly, the behaviour feels very controlling to me and it's something I would not tolerate.

Whatever age your children are they are your children and you had them first - she knew that you were a package. I would always put my children first and would make that plain to her.

Hi, and neither should you ever be made to choose! I reiterate, I don’t see how this is to be resolved without a huge turnabout from your partner. She’s drawn a line in the sand so to speak, you quite rightly want to see and spend time with your children without having to navigate her moods. Your partner is with someone with children, Mothers and children come as a package, adult children from time to time visit and spend time with their parent. You must have the freedom to have your children visit without this carryon from yr partner.. Heck, as above I do wish you luck

Yes. We have discussed how this makes my children feel. She says nothing when I try to talk about it.
She says I do too much for my children. And I probably do but that’s my choice.
She’s never had children. Didn’t want them and there’s nothing maternal about her. Although she’s ok with the grandchildren coming to stay.
I think she has some kind of social anxiety as she’s just not good with people. She has no real friends of her own and is quite content sitting at home doing her own thing.
I lovd my relationship with my middle son. And I will not let her change that.

Wow! I could've written this in the sense that my son has also had relationship breakdown, is staying with us for Xmas. He is also coming to stay with us for a short while, as circumstances are he is also looking for a new home and job. He sounds very similar to your son in that he's polite, quiet but his room is a bit untidy.

The massive difference is, that whilst we all know it's going to be an adjustment for all of us, my husband has been nothing but supportive about the situation.

We like our quiet life and routine, but as we see it when your kids need help, you do what you can to support them.

From what you say, to me it comes across that your partner could be jealous of your relationship with your kids.
Have you discussed how they make your kids feel?

I really hope you find some resolution.

The relationship is absolutely fine when we are on our own.
It’s ‘our’ house not her house.
She likes things to remain the same. Doesn’t like change. And doesn’t like her life to be disrupted.
I told her ‘he is only here for the Christmas period’
Her response ..’I don’t want him taking over my life. I’ve shared my life with your kids at some point. If it’s not one it the other’
Well I’m sorry. I had three children when you met me and I’ve still got three children who aren’t going away any time soon.
I’ve been awake all night (she slept on the sofa) throwing the word Divorce around in my head. Obviously I don’t want it to come to that but I will not be made to choose. They are my children.

What an awful situation. I don’t know how this will be resolved with your partner as you and your children are kin first a foremost. You don’t say how enjoyable the relationship is when your children are not around, but the anxiety that must be felt when one or more of your children want to spend time with you their Mum, has to be worrying. I wouldn’t like to be putting up with her moods, ok it’s her house, maybe time for you to get your own place, somewhere you have more freedom, because as I read it, you don’t have that at your partners house. I fear this situation is not going to change if nothing changes. Good luck.