I used to get like this. Hell, I felt like the detectors would go off in stores even though I paid for everything and then felt guilty for hours. It's obsessive thinking in one of it's more self sabotaging ways. And it can be really hard to break, and I am sorry it's been bringing you down.

I find it helpful to write these kinds of intrusive obsessive thoughts down in a notebook dedicated for that purpose only. Say, for example, I went shopping and convinced myself I some how stole something and didn't get caught, I'd write it down. In detail of how I was feeling, all of the negatives, etc. On the page opposite; I'd write down why it is all fine. I'll actually use an example from my notebook about this situation so maybe you can understand more because I can be terrible at describing things like this;

"I did not steal anything. I paid for everything. And if I don't believe this, I have the receipt to prove it. I have done nothing wrong. I am not guilty of anything. I am okay. This is okay. It is just thinking. I need to work on it."

From there, I was able to see what triggered these kinds of obsessive thinking and begin working on them from the ground up. Self doubt, guilt, frustration, self hate. I worked from there.

You did not do anything wrong, I promise you. You are not a fraud. Even if what you did was a shot in the dark, you still preformed wonderfully and worked for that grade. I know it is hard convincing yourself of these things, but it does take some time. And once you begin working from the bottom up, it will get easier.