To be really struggling with my dad's new relationship?

I think this is a good idea and a mature way of making your father aware of the impact this is having on you. Yes, you are an adult but you've been used to a situation of happy companionship between your parents that has abruptly come to an end. It's a shame your father didn't have the guts to tell your mum face to face that he was moving on.

Might be worth a conversation with your father that while you are happy that he’s happy - it’s a huge change for you from having your parents travel and holiday together to him moving on with someone new.

It’s ok to let him know that this is like a recent separation for you.

and plan to spend Christmas as separate families.

I think you need to reframe it and think how lucky you've been that they've both stayed so close over the years, you could have had decades of them being separated. You're an adult now so able to deal with it better.
I think it's nice when ex couples stay so close. It's the estrangement ones I can't understand when people have spent so many years together (unless abuse etc)
But I know it still feels sudden to you, maybe ask your dad to not mention her for a while?

So the mum had no say in it?

Give over. The op's mother has agency over her own life. She chose to continue acting as though they were still together. No one forced her to remain so enmeshed with him.

Wow, that was selfish of your dad to keep your mum around if his intention was to find someone else.

Why is this all on your dad, your parents are adults with independent thoughts and feelings

If your mum didn't want to be with him she didn't have too, it suited the both of them

@Didimum has absolutely nailed it. Your previous set up sounds horribly enmeshed and really problematic. Just because you’ve pretended your parents hadn’t split up does not mean he’s doing anything wrong in now forming a new relationship. Don’t punish him or his girlfriend for a totally legitimate relationship or you may end up ruining your relationship with him.

You’re not being unreasonable for feeling this way, but I think it’s important to realise that it was a problematic way to go about their separation in the first place and if either of them were to find someone special again (which is completely normal) that this was always going to be inevitable.

To be honest, I think you should feel proud of your dad for treating his new partner with respect and consideration. It shows he’s a decent man. Can you imagine dating someone who’s carrying on with their ex wife as if they’re still married? That’s not ok and your dad is handling it correctly.

I understand your mum must always be sad and bewildered, but again, it was likely inevitable. She’s an adult and should have realised it was never healthy to have this sort of dependency on her ex husband.

Your mother has made choices, too, op. She chose to continue playing happy families with your dad even though they divorced, which is just bizarre to me. They have both made poor decisions, and it was inevitable it
would all go sideways at some point. I'm surprised it took this long, honestly.

As for your dad, you don't have to be happy for him, and you don't have to condemn him, either. Just leave him to it and encourage your mother to move forward and not go back to that weird charade should your dad's new relationship end.

I think you're both old enough to speak the truth - tell him how you feel, honestly. It was a strange set up with your mum and you are 100% allowed to feel sad and like they're splitting up all over again. He's not being fair on your mum but that's not your problem to solve. I'd just be honest with him.

Id tell him to grow up and not rely on you to sort things out with either your mum or the new woman. He needs to cut the ties FGS after this length of time. Talk about having your cake and eating it! Yes your mum will be upset but sutely she realised the way they were was bonkers

You're both lucky you had an extra 15 years to be honest.

It must be uncomfortable for you and your mum and it will take some getting used to but kids / teens and adults have to go through this all the time.

Imagine seeing one parent at Christmas age 5? It's awful but it's so common.

It's natural it feels odd after all this time, but your DF hasn't done anything wrong.

It’s as if your parents have just split up. No wonder you’re upset. Give yourself time to come to terms with it. X

They've only been together four months, Christmas is 3 months. They might not even be together at Christmas.

But really I'm fuming and like the little child inside me is so upset

Google John Bradshaw. He's done lots of books and I see there's some Youtube videos about healing the inner child.

Yep.

Did you already post about this recently?

Yes, it's a great romance at the moment and your dad feels like a teenager (and is possibly acting a bit like one too) but that doesn't mean the relationship will last very long, so I don't think you should feel pressured to meet her or make her part of your life, which might be even more upsetting for your mum. Just wait it out and wish your father well without getting too involved.

oh i'm sorry.. i am actually the 'new woman' in a scenario very similar. can i suggest you have a conversation with both your mum and your dad, neither is at fault but maybe didn't realise what effect their companionship has had on you.

this new relationship is only 4 months old, so very early days and perhaps too soon to be meeting and playing happy families with, and if it doesn't work out, i'd maybe suggest that your mum and dad don't fall back into companionship again because its confusing for you.

its likely that 1 of them, and i'm going assume mum is still in love with your dad and if they 'pick up' again.. eventually someone is going to get hurt.

if that makes any sense...

Do I pretend for him?

I wouldn't. You are understandably upset by this change in dynamics in your family.

You feel discombobulated.

I would be leaning more towards my mum to be honest, she must feel rather rejected.