This is something I deal with as well. When someone holds a door for me I almost always completely tighten up and am unable to say thank you, and instead I end up nodding my head to acknowledge. I think it has something to do with an unexpected disturbance in what I perceive as how something is supposed to play out and it throws my brain into a spin. It really sucks because we can’t help but overthink it in every possible way, but try to remind yourself that the person doesn’t understand what you have and that isn’t your fault.