I don't know, I think I would not manage to wait the whole month because as you said, in a month you could feel totally different and have no need to show her the letter anymore... I think, I would send it now but with the assumption that now she is on holidays and she won't read it, and that maybe she does not like the idea of sending her e-mails but then I guess she will tell "me" when we see each other in January... I always assume the worst case scenario so then I can be nicely surprised

But you know the best how you will feel, if she does not reply to your e-mail... Can you handle it? Would you feel better if you don't send anything or if you send but won't get any response till January?