Christmas is a nightmare, when you are grieving. It is so hard to see anything but happy people all around still at a point in their lives when it is something so genuinely fun and special, but in fact, you are indeed not alone in feeling as you do. There are lots of us out there in the same boat sadly.
I lost my mum in 2017 and am also alone. I have a brother as well, but he has his own family and spends it with them. I have spent Christmas alone each year since losing my mum. I know some people cboose to ignore Christmas and just pretend it's not happening, but I didn't think that was realistic. You can't really get away from it and in the end, it is only a few days. I decided to try to make the best of it and although it's not an easy time, in my experience, it is better to acknowledge it somehow.
Christmases were always a big deal in our house when I was little with lots of visitors and food and fun. I choose to remember those times, and I do have a tree and dig out the old family decorations. I find it cosy to sit with just the tree-lights on in the evening and use that time to remember those good Christmases of the past. I also try to get some nice food in, almost in memory of my mum and dad, who always enjoyed their Christmas food and it's really not so bad. I suppose it's what you decide to make it.
All the suggestions that Emz and Karena have made are good ones and I hope you find something among them to help you. But one of the surprising things that helped me I discovered after my dad died and my mum was still with me, some thirty-five years ago. We decided to take flowers to my dad's grave on Christmas day to try to make him a part of our Christmas and it was actually a really touching experience. The cemetery was more busy than we had ever seen it with lots of people there bringing flowers to their loved ones and spending time remembering them. Some of the stones had been decorated with Christmas decorations and there were so many families there to honour and remember their lost loved ones on Christmas Day. I have never seen so many flowers on all the graves. It was really lovely and really brought home to us, just how many people there are out there who have lost someone they love and that they all still feel that love and wish to mark the day by including that person in their day in some small way, even if only by visiting their grave. We ended up looking forward to going to visit the grave on Christmas Day each year and now my mum is there too, I still do it and have a little chat with them and knowing they would want me to enjoy the day, I do make myself a special meal and try to make the best of the day. That's what they would want.
You can't recreate the past, but for me, it would be wrong to ignore it and isn't what my mum and dad would want, so I make the best of it, and have a tree and take flowers to the cemetery and go to church and sing the carols (my mum loved to sing the carols) and have a nice Christmas dinner and watch some telly and ring some friends for a chat and watch some TV and if I do still shed a tear or two when the lights are off and only the tree ones are on in the evening, when memories of Christmas past seep into my consciousness, that's ok too. I wouldn't be without those memories and thank god (and mum and dad) that I have them and my quiet little Christmases are my own little nod to those days gone by. It's fine to have fun and it's fine to be sad, but it's not fine to punish yourself by pretending Christmas isn't happening. It will happen every year for the rest of your life and it will always be hard, but you have to find a way to make the best of it that suits you and that is what those who loved us who have passed away would want for us, so it is a tribute to them, to do ourselves what they would want for us but are no longer here to provide.
Whatever you choose and that feels right for you is Ok. Go away if that helps. Find your own new traditions. But I find it doesn't help to just be miserable and pretend it's not happening. Perhaps that will be right for you, but I doubt it somehow. Optimism is always more helpful, I think.