I know what you mean, Srebak. It's like trying to make yourself fall asleep by thinking about it. The more you try, the less likely it is to happen.
It's hard to figure out why your mind is doing what it's doing through 2 forum posts (Welcome, by the way!) but maybe you're feeling the pressure that often comes from the season? (Am I ready? Did I do enough? I feel bad because I couldn't get everything? etc) Who knows???
What I suggest is to let the past be the past. Hard to do but over time, you can train your mind to do just that. I promise.
I've done some crappy things in my past and I sometimes think back to those times...and I start feeling bad. But then I realize that it was those experiences that made me the person I am today; far from perfect but better than I was back then. Just understand that those damaging moments from the past are there in part to teach you something and to make you stronger. Nowadays, I try and make up for those events by helping others who are going through the same thing.
Again, I don't know your specific situation but maybe Jeff's response (although a bit preachy. HA!) isn't such a bad one. Forget about yourself (even though you don't feel Christmas-y) and try and make Christmas better for someone else. I once faked a Christmas-y attitude to try and lift the spirits of those around me back in my homeless days. Once others started getting in the spirit, I found I started to, as well.
Don't mean to rant. I feel for you, for certain! If there's anything I can do (that doesn't include mailing you a kidney or a box of kittens), please let me know!
Merry Christmas to you whatever happens!