Hey Anjali! 

Greetings! 

Don't doubt yourself buddy,. ..it happens with everyone in the starting ages. I will help you how to cop up with this. 

Here it is,

1. First study its theory from textbook like HCV,make short notes if you have ample time.
2. Then do numericals of HCV.
3. Then again revise theory in 15 minute from DC pandey book. 
4. Do numerical of DCP.
5. Do not do Jee advanced problems at first.
6. Complete jee main first,then try for jee advance.

This is the way I used during my JEE years. 

Please upvote if this was helpful to you. 

Thank you and good luck. 

Don't worry it happens with all.

Be motivated.

Yes you should refer other books If you are not understanding the concepts.

Understand the theory properly first.

Remember devote more time in brushing the concepts

There must be clear understanding of the question first 

I would request you to refer to  the concepts again