I am this way as well. I find it easier to say thank you to a complete stranger (eg. a waiter bringing you a glass of water) than to someone I actually know and interact with. I've spent a lot of my life trying not to speak at all out of fear of embarrassing myself, so when I visit my friend and stay with his family he has to constantly remind me under his breath to say hello and good morning and such when his family members say it to me. It's a really painful position to be in, afraid of saying something foolish, but also afraid of offending by NOT saying something.

I imagine nobody will see this since it's several days old now, but I just came back from visiting said friend and it felt really relevant. I'm really terrible at expressing happiness IRL (I like to say that I smile on the inside) so I always am afraid of being interpreted as the sort of person OP hosted even when I am genuinely enjoying myself.