Work advice...seriously about to quit my job.

Some of my long standing MFPals know all about this but I was promoted without a title or pay about 1.5 years ago. It was a temporary fix that became permanent. I'm essentially the assistant supervisor of a call center in addition to processing bill & corporate payments, trouble shooting other department's problems and generally being the manager's b*tch.

I have pleaded my case time and time again about how I've learned the job and have been doing it every single day for almost two years. I've had the carrot of "retroactive raise" dangled in my face.

NOW the newest person in our call center quit and my boss is just assuming I'll be picking up his Saturday shifts. I have my limits and I will not do this. I was JUST taken off the Saturday schedule a few months ago.

How do I approach this for the FINAL time? I need to make a stance but I also can't afford to quit right now without anything lined up.

Any advice is appreciated! Thank you!