Devastated about the maths

You do not really say how you have studied!

If you just glanced at material without learning technique and practicing on questions and exams that are on the proper level, this may account for your performance. I do not want to be presumptuous though. There is no reason for a student who puts in the proper work with the proper material, to fall in the quantitative section (especially the quantitative section). You have done very well in the Verbal (nice jump) but again, without seeing how you work, understanding your work method, or knowing where your weaknesses and strengths are, it is difficult to diagnose what is going on.

I think you will need to be very very systematic in your approach, meaning, I believe you will have to start from the ground up - I don't mean learning maths from the basics but learning GMAT maths from the basics in a more personalized manner. Obviously something is going on and maybe you simply need a better curriculum, one that addresses only your needs. In essence, the knowledge gaps which you seem to have, need to be dealt with. Once you internalize the techniques and learn to avoid the traps, it will be very important to be able to deal with the highest level Quant questions. But first the proper lessons!