I used to be exactly the same, before I had DD. My Dad died on Christmas Eve when I was 11, and Christmas was never the same after that. I never even had a Christmas tree, when it was just DH and me, but when we had dd, I decided it was time to start enjoying. The first few years were very tough, but tbh, just having a child in the house, really does help. Obviously, since I was only 11 when he died, I had a lot of years between then and DD arriving, so I had done all my grieving and I had just got myself into the mindset that Christmas was always going to be crap.
Why don't you start some new traditions with your family? You will never forget the Christmases with your Mum, but its not nice for your LO to hate it because of you. (I'm not being rude when I say that, remember I know how hard it is!)
One very important thing that I do on Christmas Day, is raise a glass to my Dad and actually a lot of the time, its DH that starts the toast. My dad would have loved his GD, she's very special and I make it special for her. Your mum wouldn't want you to be miserable for ever and tbh, she'd probably tell you off for not making it fun for her grandchild!
You are not alone in feeling like this, there's 3 of us on this thread for a start, so stick with us and we'll help you through.