Feeling worthless. Is increasing daily?

10yrs ago I used to be an average student. No one had any expectations from me. But then I suddenly surprised everyone and achieved what I ought to. But it didn't stop there. A lot more is there to reach. But I'm back with my average or below average level. Now everyone wants to see me achieve something, including me want something to do for myself. I convinced myself all these years that I can achieve things I wanted to. But people around me are far better in understanding the subjects, applying the knowledge to solve questions and getting better ranks. I too wanted to study the same way but how hard i try, I'm not even able to concentrate. It's been 2 years since I've been stuck around with my books but nothing changed. Maybe I dreamed too high for myself, that I now am not willing to accept where I am , what I can do. Now recently I'm crying whenever I'm alone, easily get distracted,forget that i was crying and again back to crying one's distraction stopped which is increasing daily