First of all, from what you mentioned about test, that's test anxiety, and many people, including myself, suffer from it occasionally. The best thing to do in these cases is to take a few deep breaths, visualize a relaxing scene and when you are ready, start.
As for the part about not believing in yourself, that takes time, and having a good support system helps. You should try stating a strong "I" affirmation in the morning to help boost your self esteem, something like "I AM GOING TO HAVE A REALLY GOOD DAY TODAY" or "I AM BEAUTIFUL"; "I AM INTELLIGENT" ....etc...etc..
These I affirmations not only help boost self esteem but you'll start feeling like a totally different person who is confident in their self and you'll start gaining a bit of pride along with it. I used to have very low self esteem ( hunched over, wouldn't look anyone in the face, would sit up straight etc...) but with positive reinforcement from myself, and from my now husband, I've managed to become very confident, proud, and actually sometimes a little overbearing, lol, but all in all I don't intimidate people anyone by not looking at them (although I still can't look anyone in the eye directly), I walk straight, sit up straight, and just have all the charisma I used to have a child, return. You can be a very self empowered individual if you work at it.