I'm very glad you have taken on board what people have been saying, and are considering talking to your Ex. I DO understand how hard it is to be a Lone Parent with a disability. I have uncontrolled epilepsy, and I have four dc to look after, two of whom ALSO have disabilities.
I DO understand how you feel, and it IS heartbreaking not to have your child or children with you at Christmas - but what ends up happening with my dc is that, in essence, they get TWO Christmasses EVERY year. Which, as far as a child is concerned, is the best thing ever. DS1 gets Christmas on Christmas day, then he gets Christmas on Boxing day too!
It won't seem like it now, not the first few times your dc is away for Christmas, but you do, in a way, get used to it. DS1 is lookng forward to spending Christmas day at his SM's parent's house, and he is also looking forward to spending Boxing Day opening presents at my house. He gets a traditional Xmas dinner on Christmas day, and a huge buffet on Boxing day. Then next year, it will be reversed.
He gets to experience the Christmas traditions I hold and want to pass on, but ALSO those of his dad's family. Like having disgusting bread sauce all over your dinner.
When DS1 comes home from his dad's with his face all lit up, with stories of what he got, and how his brother pulled the Christmas tree onto the table and they had to pick pine needles off the turkey, and then he is bouncing up and down hugging me and bouncing around the front room waiting to open his presents here, I know it has been worth it, because HE is happy.
Men can be utter shits as partners, and still be a good father. It just needs the pair of you to put aside your differences. As my Ex-H's MIL has to work on DS1's SM's DD - I may well be the one picking her dc up from pre-school and school when she has Ex-H's new baby. I REALLY didn't envisage THAT one 7 years ago...But, when all is said and done - they are just as much DS1's siblings as my own DS2 and DS3 are.
Doesn't mean that DS1's SM isn't a total cow, mind you. I'm still human, after all. 
You can internally seethe loads at your Ex and his DW - just as long as it doesn't affect the way you TREAT them or act around them - be as two faced as you can, because ultimately, it is to ensure your DS's long-term happiness and well-being. Just come on here and moan when they make you want to stick pins in their eyeballs. 