I'm sorry that totally sucks, u not getting the right cheese from the shop cannot be attributed you "can't do anything right", it doesn't say anything about u, merely that u got the wrong cheese. Your mother is being really harmful, does she know she's doing that? Would it make any difference do you think if you said, "hey calling me a retard really hurt my feelings, i was trying to be helpful getting the cheese, but your response really made me feel that I am not appreciated and I am unworthy. And when it happens so often it really makes me feel like you hate me and wish I was never born. Can you please think about it and think about how your actions is making me feel?" I know it's a long shot. I'm really sorry u r going through this and have done for a long time to make u feel this way. I hope u can find a way to get through to her. But if not, it isn't your fault, u have to protect yourself against the harm that she is incurring on you by probably just stop expecting anything from her and just live your life as if she doesn't exist