I don't know if this will help. I didn't get my license until I was 23 years old. I had a hard time with the test too. I do not believe that you are dumb. Maybe you are trying to hard. I feel that you are being to hard on yourself. I do the same thing. I have to get up in the morning and look in the mirror and tell myself I am a good person. It is very hard to do, but I do it. I use to cry alot alot. Sometimes not knowing why I was crying. You need to believe in yourself. I have found out failure is a part of life. That is a hard bitter pil to swallow. IF you could look at failure as a area in which you need to improve, maybe that would help. I get up and make a list of things that I need to improve in. Sometimes I only accomplish one thing a week. I look at failure now as a challenge.
Good Luck I Know You will get your license.