Hey there, first of all, I am sorry you are feeling like this, I can understand that not having answers and the insecurity about the relationship isn't doing much good to you right now.

There is something called the Honeymoon Phase in every relationship, it's the phase where everything seems perfect, fresh and exciting, and you're constantly learning new things about each other and having first experiences together, but after a while you shared everything with each other, you fall into a routine, you don't feel the need to text your SO constantly anymore, you are getting to a new stage of the relationship where you are really comfortable with each other and you expect your SO to know how you feel about them and trust in your relationship.
Sometimes partners can disconnect during this phase because for one it can feel like everything is going fine while for the other (who is still in this honeymoon phase) it feels like panic.

But, when I read your story, I do notice this, he did ask you to come over for Christmas to spend it together? If he wouldn't be interested in the relationship anymore he wouldn't ask you to come over for something that is 5 months in the future.
Maybe he is facing problems in life that has nothing do with you or the relationship, maybe problems at work, school, family or friends relate.
Sometimes a partner needs time to disconnect to think things trough, but I do feel you shouldn't be suffering for him to be able to do that, he should be able to communicate this with you and tell you what's going on and that he needs some time, communication in a LDR is so important, without it your SO is left completely clueless about what's going on and that usually makes the mind go crazy and leaves room for speculation.

What you can do is try to ask him a question without making him feel attacked or accused.
for example:

Hey, I noticed you have been acting cold and detached recently.
I wanted to ask you if there is something on your mind or something going that I can maybe help you with?
I am here for you and would love to help you with anything you are facing right now.



Maybe that will allow him to open up more about what is going on.

I had similar experience before. He suddenly become distant and didn't messaged or replied to my messages at all. I beat myself into thinking of possible things I did to make him be this way. After almost 3 weeks of silence, I suddenly had the courage to message him for the last time. Told him I just want to know how he is doing and that, and if he said he is okay, I'll leave him alone. Turns out he is going through some hard time. Be patient with him.