Welcome to the 90% of us who do well, are motivated then trip up and regain. You are far from the exception so please don't beat yourself up about it. In fact, you've done well not to regain everything you lost and added to it (that, statistically is what most people do)
Why not be kind to yourself, decide to have a nice Christmas without going too overboard and NO GUILT (after all, do slim people feel guilty about enjoying nice food on a special occasion? No.)
Make a conscious decision to allow yourself that (but as I said, practice moderation) then, make another conscious decision to start 2012 with a new, refreshed attitude and a new refreshed resolve. You'll have loads of company as there will be an influx of people starting new diets in January.