Like they say,ifyou've never failed...you've never lived. Don't loose hope, every big achievemnt was first a failure.
I am myself no achiever, just gave the gmat n messed it up under pressure.
However, I would recommend few things to start with :
1) Maths: Use manhattan guides, they start with very basic concepts and then will make you reach a level where you'll be able to solve 85 % of GMAT questions.
Trust me, they are very helpful.
2) CR : I used Manhattan CR
book for CR as well, however I feel a book cant really teach CR.
I read in one of the post which recommened that we need to know the conclusion first, check whether question asks us to strengthen/weaken it and then accordingly think in mind what could have supported/weakened it.
Then read the options and see which one is the closest you were looking for.
3) SC : I read Manhattan SC, a very good book, you should use it too. However, SC initially was tough for me. I started scoring an 80% accuracy only after I started paying more attention to the sentence and then analyzed what its talking about. Analyzing the sentence n understatnding its theme helped me a lot.
For SC n CR refer to old OGs , dn;t focus on speed initially, first focus on accuracy, one you have accuracy, go for speed.
4) RC : I am myself struggling with it a bit. Can't really help.