Am i the only one struggling to say thank you?

I know that it's so easy to say thank you but for me i can't say it especially to my relatives, i don't even know why i can't say it so they got angry or disappointed to me when i did not say thank you to some relatives they sometimes said that"is that to hard to say thank you? What a shame, you're so stupid" that's what my uncle used to say to me which made my eyes tear up, But they were actually right..