This is a list of tips/advise/suggestions I have drawn up based on my own experience - both while I was a student preparing for the CAT as well as the years at IMS where my job brought me into direct interaction with hundreds of MBA aspirants. Some of these might not apply to you directly, and if that is so, ignore them - please don't try to find convoluted ways to connect them with yourself or your experiences.
1. "I am not IIM Material" - Thousands of students, every year, for one reason or the other manage to convince themselves and more unfortunately, many others that they are not "IIM material". For god's sake, please understand that there is no such thing as "IIM material". The worst thing which happens when you make this decision is not that you don't make it to one of the IIMs but that you will end up in a B-School not even remotely as good. Leo Burnett, the famous ad man once said that "When you reach for the stars, you may not quite get there, but you won't come up with a handful of mud either". So aim for the IIMs - even if you don't make it to India's best Business School, at least you will not end up in one of those fake b-shops.
2. "I am not good at Maths" - The CAT, MAT and most of the other Business School Admission Tests are designed to test your aptitude for a career in management, not your maths acumen. Yes, if you are good at Mathematics it will help. But the vice-versa is not necessarily true. A few of the questions in the CAT might require a high level of mathematical acumen, but then you need to get less than half the answers right to make it through, don't you? With sufficient preparation, you can crack enough of the questions in the Quantitative Aptitude (QA) section that can help you sail through. Remember that not every IITian who writes the CAT makes it to the IIM - they do better than others because these guys are good, diligent students - else they would not have been at the IIT. So please don't let the 'maths' thing discourage you.
3. "I shouldn't waste another year preparing for the CAT" - Didn't make it to a top B-School in CAT 2008, gave up, and joined a mediocre B-School because he/she didn't want to waste another year preparing for CAT 2009.
I don't have any empirical data handy to substantiate what I am about to say so I would recommend you cross-check this fact with a few veterans.
"Students rewriting the CAT have a higher chance of making it to an IIM than those writing the CAT for the first time." We are all human beings, and we make mistakes, more when we are trying to do something for the first time. We make fewer mistakes the next time because we have learnt our lessons. Even a maestro like Tendulkar took a few years to sort himself out before he reached the peak performance level. There are a few lessons that you will learn while writing the actual CAT that you will never learn no matter how many mock CATs you write.
And as for the idea of "wasting a year", it's the most stupid thought that should have ever occurred to you. Please understand that the kind of B-School you will graduate from will very likely decide the quality of the rest of your life. If by "wasting a year" you can greatly enhance how you are going to spend the next 40-60 years, then it's not wasting but "investing a year".