not being able to say thank you

not being able to say thank you I struggle a lot with 'th' sounds at the beginning of words. So when I am ordering food, receiving compliments, or doing anything that requires me to say thank you after, I stutter. And some people just think I'm being disrespectful or that I didn't hear them because it takes me so long to get the word out. I hate it so much. I feel so stupid and the other person has to wait for me to say two simple words. It's so frustrating and I feel bad not being able to finish the interaction quickly. Sometimes I am just so sick of myself and this problem.