The worst for me is not being able to say "thank you". I can usually manage to spit it out with a different tone of voice for one-off instances like someone holding the door for me, but in more personal situations like a professor helping me with a problem or something, I really struggle. My options are either look like a jerk and don't say it at all, or waste everyone's time taking 15 seconds to get it out. Usually I choose to push through it, but sometimes I just don't have the energy and can't bring myself to do it, and that really hurts. In your case, I might suggest adding a sort of "buffer" to the beginning when you're in these situations. I find that sometimes if I say "I appreciate it, thank you" instead of just "thank you", it makes things smoother. So maybe if you say "oh, it's okay" or "ah, don't worry about it", you can bypass having to start with a "real" word and get you going quicker. I have no idea if I explained that in a way other humans will understand but either way, good luck!