If you have doubts that Jesus is God, then you cannot truely celebrate the "True Meaning" of Christmas because as you yourself said you recognize Jesus as being only a prophet, and that it is just a prophets birthday. Can you pertake in the celebration with those who are true believers in the spiritual meaning of Christmas? Of course you can, but you won't be doing so for the same reasons that they are. Religious meanings of Christmas and their sprititual beliefs of Christmas.
I don't need someone else's approval or their analysis of my spiritual state, in which I celebrate. We could have doubts and disagreements about the qualities a person has, and we both can celebrate his birth. You might not approve of what I believe about him, and I might not agree with what you believe, but I too can have my share of spiritual celebration just as you can, even if it is different than yours.
Remember, we all have been told what we believe in and I don't see how anyone can be 100% sure which one of them is the right one. I'd rather see people gather together for a good cause, than be separated by those who analyze other's beliefs. Yes, I don't celebrate for the reasons you do. Who's there to say which one of them is the true reason and which one is not?
"Fire cannot kill a dragon." -Daenerys Targaryen, Game of Thrones
