My lover's parents think I'm rude- what should I do?

My boyfriend of over a year recently told me that his parents felt I was rude last time I visited. They weren't happy with me making a cup of tea for myself without asking or for once leaving the table without excusing myself. I see these as pretty minor issues although I understand where they are coming from. 

My issue is that I try my very hardest to be polite to them at all times and always have. Knowing that I have failed at this and that my boyfriend's dad is pointing these things out to 'help' me (as he said) makes me feel immature, stupid and like a failure who has reached adulthood but still doesn't understand basic manners.

I don't want my anger at myself to lead to any defensive behaviour around my boyfriend and his parents or any other actions that could compromise my relationship with them. How do I move on from this and stop these feelings of failure from eating me up?