Why can't I concentrate and remember things?

I have a lot of problems with listening to others because I can't concentrate. I think about other things while they are talking. I also make simple mistakes that shouldn't be made. For example, I am a cashier, and I hit the wrong button on the register, and I knew what I did wrong right after I did it, so I called a manager to fix it. As soon as the manager left I hit the wrong button AGAIN. Am I just crazy and stupid? Its quite embarrassing. How can I ever have a real job if I can't even remember something from two seconds ago? I feel like my mind is all over the place, and no matter how hard I concentrate I just can't focus. It also affects my speech, because I want to say something but can't think of the words to say what I want to say. Sorry for the rant, its just very aggravating.