There is no cure so no one can answer it but here is the thing, in your mind you think people what, looked down on you for it?  That you were embarrassed?  That exact opposite is true.  They felt bad for you, thought you were a sincere nice person trying to say thank you and most probably wanted to hug or pat your shoulder when its as difficult as you say.  So if you stop worrying over it thinking it effects you so negatively you may be surprised that even though there is technically no cure, you may be able to say thank you in that situation easier and easier over time without worrying about it to where you don't stutter on it anymore or its so rare you don't care.