To have resented our Christmas routine for almost 20 years?

Do you not have any other family on your side? I think whatever you do, you can’t change the past so just have to accept the annoying nature of what has been. Then announce what you would like to happen next year and stick by it. You could say you are going to your own country or that you would like to have Christmas somewhere else/go on holiday/go out to eat or whatever. Or simply insist that next year your BIL hosts MIL. It’s going to cause upset and consternation. Blame the menopause and stick to your guns 😂. The alternative is saying nothing and spending the next 5-10 years exactly the same. Also bear in mind that DD as an adult may start to spend Christmas away anyway which will change the dynamic.

I sympathise OP, I really resent our routine too but don't see a way out without upset and upheaval. I've said it annually to Dh that I hate it but nothing changes, it's an issue with both sides of the family and elderly parents. This year I got so stressed I almost had a panic attack when they went home, it's crazy it's only dinner but I have so many negative stressful moments from Christmas it seems to be cumulative and gets worse every year so I anticipate the stress and get myself worked up over nothing.

You need to start by talking to your DH now while it's fresh in your minds. It's ok to tell him you don't like it, you should trust him.

So your DH will be able to cook a Christmas dinner for himself and DMIL (from what you've said). So you and DD can go out for a lunch next year. Sorted. She won't be on her own, she'll have her son.
If you don't break the cycle now, when your DD gets her own place and wants to host or has children or whatever, you won't be able to go.
Stuff BIL, he can sort himself out

Yeah I agree with this. It’s probably too difficult with all the F.O.G now to stop having MIL over - but I would do everything in my power to insist the meal happens out or your DH and BIL cook it. That you can change - they will resist but you have to refuse.

With your MIL being so elderly, realistically you probably won't have many Christmases left with her. Maybe try to enjoy spending time in her company while you can but think about starting some new traditions when she's no longer with you.

Well things will change if DMIL gets ill/ DD has a partner or baby or any life event could change things so dont despair.
I would try to do something -take yourself for a walk, arrange to meet up with friend for a half hour break. Plan a movie you want to see. What else could you do when they’re at church - must be something ….

Lots of people don't enjoy Christmas and just go through the motions as they think it's nice for others. YABU not to have said anything for 20 years but I don't think expecting Christmas "off" is reasonable either. You could include foods and activities you like. Do that from now on.

Complaining about being left alone whilst others go to church is unreasonable as its literally the name of the day: Christ - mass. If someone in my family resented my tradition of going to church at Christmas they could do one.

Bollocks to that. This year, announce that next year things are going to be different and state what you’re going to be doing and say anyone is welcome to join you.
(Alternatively tell them you’re going away for Christmas, that’s what I’m doing!!)

You’d get much more sympathy here if you blamed your husband. There are posters practically begging you to.

It’s like pantomime. They need to be able to boo someone.

You could definitely have said something and change it up each year eg go out to restaurant, cook something easy, host at a different time or day, share the cooking with other family members etc. When I suggested a change from the norm with my family as it was becoming hard to be the one hosting, everyone thought it was a great idea and we all do our own thing now.

I think you are unreasonable to feel resentment when you've just gone along with it and said nothing.

Sounds like a 'proper' Christmas to me. Even if it's not ideal, its only as you say for two days.

I left after realising I'd have to do a Sunday roast each week for 2pm as he arrived back from the pub hours later.

Oh what a mummy's boy

You could still go for a walk? Surely Mil can be left alone for a bit to have a nap/watch tv

You should have spoken up years ago, I'm afraid. It's a bit late now when your MIL is so elderly.

Unfortunately, the time to say something was when you realised that your husband expected the same format, year in and year out. Your BIL has never bothered to host, and is never likely too either. Your FIL has passed away, so your MIL is on her own. Your MIL coming for Christmas is going to happen now, until she passes away. I don't think your husband will be open to changing the format, as he sounds rather stuck in his ways. However, if you don't say anything to him about going out for lunch next year, then he won't know how you feel and nothing has even a possibility of changing. Your husband isn't a mind reader!!

I hope your husband did the vast majority of the cooking, cleaning and hosting on the day itself?

When they laughed you should have said ‘I mean it, I’m too tired to cook for everyone this year so it’s either eating out or not eating.’ Then you stand by it.

I mean I don’t really see why you’re resentful. You didn’t ask to do anything different and Xmas is generally about family which includes his parents. After a certain age most people do not host - my in laws transitioned this to me when FIL turned 70.

If you want to change the food then do. If you and DD want to walk without MIL then do.

When you say 'we host..' does that actually mean you do all the work? Does your DH gets involved or just talk to his mum?

This year, at dinner, say 'we've had lovely dinners together for 20 years so next year I'm giving myself a year off cooking. Where shall I book for dinner?' And mean it.

The time to say something would have been years ago. Even if that was 'should we do Christmas Day lunch at your house next year' or 'Brother in Law I could really do with a break next year as I find lunch etc so tiring - could we all come to yours'.

You don't know how long MIL could be around for (a relative of my Husband's lived until 99) so you could have many more of them. Maybe suggest a restaurant next year?