It seems you put that thought into your head associating his christmas gift to her of last year. Let it go. It's in the past.
There will be such triggers as you go along. next time you have such a thought, examine it and see if it's worth keeping. Deflate it with reasoning on it. For example, if that thought were to come to me, I might say to myself, "that's how he felt last year toward her. Duh, of course that's how he felt at that time. But it wasn't really love, and now he's here working on things with me. He doesn't keep in touch with her anymore, and we're going to be spending christmas together."
I agree with you that you can give a token of your love at any time. IMO, Christmas isn't really about "giving people you love gifts", it's more about people exchanging gifts. Giving a gift when it's not expected is much more meaningful.
