i know this feeling well.
luckily, in the UK, the 26th december is like another christmas day, it's called boxing day and we celebrate it. everyone seems to keep their decorations up until atleast 1st january and christmas films are still on tv for a few days yet, so that keeps me going. i still feel very festive until new years day.
once january comes i get sad, but i start thinking about the next christmas and what i'd like to do differently and how amazing i will make it. coming on the forums helps and i plan to start my official build up a little earlier in 2012.