If it were me, I would wait until after Christmas. Not only will this give you time to get all your ducks in a row, but no mutal friend/family member/late night phone call gets to throw "You ruined Christmas" in your face. Sounds feeble, but that's just one more stressor you don't need right now.
Take the time until then to find another place to live and to put away the money necessary to move. Until then, act natural. It may seem like you are being dubious, but its far better than having a nasty fight and ending up on the street immediately with no place to go. Make sure that you are covered and provided for. You've tried to be responsible for her, it didn't work, so now the focus is on taking care of yourself. It will be a lot easier to handle her this next few weeks once you know you are leaving.
When you decide to go, tell her to her face and at a time when she can't stay and try to hash it out with you for hours. Maybe an hour or two before she has to be at work. This way, she HAS to leave and you have peace while you get your stuff out. Be completely gone by the time she gets home. And for God's sake, no breakup letters. Those end up posted on here all the time...its so sad its funny.
Best of luck to you.
There are very few personal problems that cannot be solved with a suitable application of high explosives.