Well there are two sides to it- one, a philosophical one, and the other, a technical one.

Technically,you are unable to understand because you don't have the intellectual capability to digest it. This is due to backlogs in most of the cases, so make sure that you don't have them. Many topics can be too tough to grasp or maybe you might feel overloaded, because JEE is meant to be like that. But keep on taking in as much as possible.

If you can't do tough problems, at least try them, listen to your teachers, see the solution, after 3–4 iterations, you will surely see results.

In case you have lots of backlogs, try clearing them on a basic level(notes and NCERT) along with your current class syllabus. Never leave the current topic. Try using summer break, winter break, holidays, weekends, etc. for clearing backlogs and revising.

On a philosophical note, it is common to feel low during JEE preparation. It is because you are suppressing so much of your thoughts to focus on one goal, and that too at this age(I am your age, so I do know how it feels that way).

But believe me, it won't go a waste. Give your best, irrespective of results. In the end it will be fine. There were lots of intelligent people in my batch. Also, many were there who couldn't get things(one of my best friend included). Scored low.

Everyone clowned and frowned at them. But they didn't lose hope….they kept on trying. And they did really well(again, my friend included who is in an IIT today).

It is important to realize that JEE rank is proportional to number of hours as well….although you may not get stuff easily, but it is important to at least try.

At least listen to teachers, discuss with friends, try doing it on your own after the teacher discusses it.

Your final rank depends a lot on the number of setbacks you get yourself into.

All the best!

You can also check out this app I made for helping JEE/NEET/Olympiad/KVPY/NTSE aspirants with courses, notes, blogs, and much more- AcadBoost