To walk away.

Where has this idea of you leaving him so that he can have the relationship he wants with his daughter, come from?

It isn't you preventing the relationship, it's her.

If you think that in the future when there are more constraints, he’s going to pick her over you and end up treating you in a way you won’t like, that’s a reasonable concern. But own it. You’ve framed things here as you leaving for his sake.

Your contention is that leaving means he can have a better relationship with his DD. But there’s nothing in his past that suggests his relationship with her will be any better if you leave. Because his history suggests he doesn’t choose to center her in his life the way she wants (and I don’t blame him, it doesn’t sound healthy).

When he gets older, they will be older and perfectly capable of deciding for themselves. They may decide to visit him in his own home - initially because they are curious about what deep dark secret has resulted in them never going there.

Once they realise there is no deep dark secret and it's just mum being weird again ( and I bet she is weird in other ways too), they will probably visit, both when invited and also off their own bat.

It’s pretty wild to reason that you should leave him because (in your opinion) he craves a relationship with his DD that he can’t have while you’re around.

He’s clearly choosing you over this relationship with his DD that you think he craves. And from what you’ve said so far, he’s chosen other women over it too, so would probably find someone else if you leave him. It seems clear he craves a steady romantic relationship over a closer relationship with his (controlling) DD.

Why would you want to override his autonomy in this? Are there other reasons you want to leave him? Is this a palatable excuse so you don’t have to confront other issues?

Does your DH have any other DC?
Could they talk to her?
Could you talk to her?
Could anyone talk to her? Her mum? Her husband/ partner? Therapy?

It just seems so sad. Unless there is a huge backstory, it seems that all of this is her 'problem' and something you have no control over.
Do you think she has the capacity to change? Sounds like she has behaved consistently with various women for a long period of time, so actually it's not about 'you' but about 'her'.
Do you live close to her?

Talk to your DH about it and listen to what he would like to do. I think leaving him over this is extreme as ultimately the problem stays the same. Is his DD only happy with him when he's single?

He knows exactly how I feel and says he won't be made to choose.

But he has chosen. You. He sees his family as much as he’s allowed as he has chosen to be with you.

Don’t feel guilty, but don’t leave. Unless you feel he is blaming you, which would be unfair of him. But otherwise enjoy your life together and let her revel in her bitterness.

But the children are growing up and in a few years won't want to spend so much time with granddad. It's almost like the DD is the OW.

Does he love and want to be with you?

Do you love and want to be with your husband?

If you end your marriage, could you afford to keep your home? The house would be included in the divorce settlement.

Unless there's some huge backstop, she sounds incredibly selfish for a grown woman. Why is she allowed to be happy but her dad is not? I think you need to speak to your DH. And someone needs to speak to her. Don't break up if this is the only reason, although I admire your love for him wanting him to be happy

OP, if you are not happy then there is nothing to keep you together.
Start by asking him to move out of your house and take a break.
Hopefully that will clarify how you feel.
She is toxic.
You are correct to stay away from her.
Thankfully she doesn't come near your home.

Could you go away for a night (somewhere fabulous on your husbands bill) every few months so he can have grandkids over. He gets the relationship, you get a fab break and keep your relationship?

walk away end your unhappiness?might as well then

I agree with @timetodecide2345 , especially this:

I think it leaves you with a sense that you're not important and somehow making a fuss about nothing. If he doesn't respect your feelings in this then I don't see that you have a choice.

It is clear that he is NEVER going to do anything about this intolerable situation. This will be your life, and I fear that, as he gets older, he might start blaming you.

Just get him to go down and see the grandkids by himself every so often. Get him to stay in an Airbnb close to her house. You don’t need to get involved or overthink this, pick your battles. Frankly i would love a few quiet weekends.

He's acted cowardly. Avoided confrontation at the expense of you.

I know where you are coming from. My DH has done this with his sister. She will blatantly ignore me and communicate only with him and our children. It all could have been avoided if he had spoken with her at the time but he didn't. I now don't respect him and want to leave.

I think it leaves you with a sense that you're not important and somehow making a fuss about nothing. If he doesn't respect your feelings in this then I don't see that you have a choice.

I think it's his problem and he should have sorted it out years ago, especially as she has ruined previous relationships. The children will get older and will be able to develop a relationship with him independently of their mother

If you are happy, stay and have a clear conscience. If you are not happy, are you looking for an excuse to leave.

Also if you left him for that reason he'd find a new gf and then have the same problem

If you love him and you're good together why would you end the whole relationship just to enable them to have sleepovers a few nights a year. If you want you could always book yourself on a yoga retreat and they visit then. But that is still doing too much- it's on your dh to find a solution not you.

Imagine being 38 and trying to stick yourself in the middle of someone else’s marriage to ruin it, and then it being your parent’s marriage. There is something wrong with her, and I would be letting her know that as my parting shot!