How do I learn to let go of what I can't change?

My parents separated almost three years ago. They each live with their respective partners. We have had a Christmas ritual for a long time, consisting of a couple of books we read every year, and then exchanging gifts. This is important to me, my sister, and especially my youngest sister – she is almost fourteen. The thing takes roughly two hours.

This year, though, our dad does not want to take part. He has invited us to his girlfriend’s house for the morning of Christmas Eve. None of us are happy with that. Our parents don’t fight every time they set eyes on each other, but neither do they have any kind of substantial relationship. They spend a couple of hours together about three times a year, at this point. Which is why Christmas is pretty special, I think, to my youngest sister.

We have had to more or less talk our dad into coming last year, and this time around, he seems to have made his mind up. He says he is sick of our “tantrums”.

Is there any way I can learn not to care so much? I’m not sleeping well at the moment, and I’m experiencing shortness of breath which is anxiety-related. I know Christmas is a day like any other, but I still feel disappointed, frustrated, and just plain sad.