well i do not think disha's material and the kundra stuff is good stuff ...
let us talk about the chemistry thing first ...
well i think u shud use 1. P.Bahadur for physical chemistry .. 2. Arihant books for organic and inorganic ...
For mathematics now .. so since u are already facing a problem ... i suggest u to the following well doing ncert is a good option to get the basics clear ... but i think u shud quickly go over this book ...
The stuff required in JEE has much different approach and the question are very different so u do need to practice a lot of stuff ... n i do not think that disha will help ... well what i suggest is that u go through what u have already done and not try much new things as it will help u grow ur confidence ....
I think that going through previous years papers is helpful only when u r comfortable with the subject and when u r clear with the concepts ... so again i do not think that u shud go for it before u have done the basics .. for other subjects u can definitely solve previous years paper .. but since u r facing problem in maths i do not think it is a good idea ... i think u shud talk about this with ur teacher in more detail ..
i hope i have answered ur queries .... take it easy .. ho jaega ...