The worries and concerns you have as a student are very normal. Learning,working hard and still remaining unable to perform as well as expected is what happens with almost everyone.
Don't lose your heart, there are thousands of students who have similar stories as yours so be hopeful. Work with full faith ,give your best .Divide your time well, plan and proceed. Study wisely. Spend some time on meditation
, it will help you to concentrate better. Have some part of the day in the company of the person who inspires you .Start afresh and be full of hope .All the best.