A bit late to this, but let me add a few points:

• Everything you did in life, all the exams, applications, tests, papers, extra-curricular activities, and everything else -- just got you the interview. Every candidate starts from scratch, the stuff before was just the ticket to get to play.
• Every candidate who gets a consulting interview is far, far above average. Typically, these people have very rarely failed at anything or gotten rejected.
• You cannot have studied or prepared too much for this. How much would a consulting job offer matter to you? Compared to getting half a grade better on your final thesis? The last exams? 2-3 months of your life? Many, many applicants study way more for one of the final exams they take, than for getting the job interview right. Which is completely wrong in my opinion.
• Case interviews can be prepared. Other interview processes in other industries tend to be a bit random, so harder to study. Here, time is invested well.
• Even if you fail the interviews, you still learn from doing the case studies. It's not a waste at all, doing cases are an intense problem solving/structuring course that will help you in almost any career.
• Hack: For top consultancies, you can always postpone the interview date. Don't do it last minute, but if you cancel 2-3 weeks in advance they won't like it but also couldn't care less if you come up with an ok reason. And only HR will know that you rescheduled, your interviewer will not.
• Lastly, the study/prep time is not about grasping the interview process intellectually. You're smart, you will get it quickly. It's about internalizing it and having it come out naturally -- for this, you need practice, practice, practice. It's like a theater play. If you go out on stage and your main worry is to remember the words, how good will you be acting? Only after you master and internalize the basics, you will be able to shine in a live interview.

By the way, in case you wonder, I failed my first interview at Bain badly. One of the case questions was whether a Swiss mayor of a small village should have a ski resort built. After thinking about it for a while, my response was: "Is there enough snow?". (true story)

Afterwards, I postponed all future interviews for a few months, and studied this thing full time. Still bugs me sometimes that I screwed up the Bain interview as I really liked the firm :)