Christina is right in that only a professional can diagnose you, but I bet you're wanting to hear general suggestions too.

When I read your post, it made me wonder if some parts of your brain just aren't relating some things to others, or just aren't as used to different ways of functioning. So for example your brain naturally handles mental arithmetic well, yet when a calculator is involved those links of thought which calculate the bill are broken so things don't make as much sense. So maybe your brain is used to another way of thinking?