Why is it so hard for me to say “You’re welcome?”

"You're welcome" means you are saying you are more than happy to do whatever it is you are doing for your mother, which you probably aren't. And I don't blame you. The whole care giving scene becomes too too much after awhile and it's only natural for resentment to start sneaking in. Right? Look into getting help and respite care so you can get planned and well deserved breaks on a regular basis. Don't get so busy caring for another that you neglect yourself, my friend.