Arts UG major interviewing at BB

I attended a top non-US school and majored in something completely worthless (I wanted to work on the side to gain practical business experience while studying something that I felt was out of my comfort zone for the challenge). Long story short, my heart wasn't in it but I graduated. I have some finance experience and am interviewing at a BB soon.

I was thinking about going back to school for an accounting degree when this came up. I'm not too hopeful about it but will die for this job to gain the experience. It's in Client Service but I feel completely out of the game because I currently work in corporate finance. I am thinking about a more quant-focused graduate program but I'm reluctant to bring that up at the interview because it doesn't seem like this position could lead into that direction.

I am worried about a ton of things and don't feel up to speed to perform well at the interview. Most pressing of all, I am not sure how to explain why I quit my receptionist job at a fortune 500 financial services job (I sent an old resume that may be misrepresented as being there for a year instead of 3 months).

What can I do to boost my game? Or is it pretty much game over?