Alone outside on a cold night, the speaker looks up at the stars and cries, then goes indoors to a warm, noisy Christmas scene and puts on a cheerful face. The poem moves from that private moment to the public one, and the gap between the two is the point.
Outside, everything is honest. The cold settles into the speaker's bones, the wind seems to say what they can't, and tears fall onto their sleeves. Inside, there are hugs, kisses and presents piled under the tree, but the feeling doesn't match the warmth. The last couplet makes this plain: the smile is "fake," and the face behind it is one "no one knows."
The mood is lonely and quietly sad. The speaker isn't angry or dramatic, just tired, and calling the Christmas "painful" suggests this has happened before. The title, "My Truth," fits: the real feelings only get spoken in the dark, where nobody is watching.