Well how have the interviews gone? As far as that goes I think you just have to keep sending out applications and going to interviews till you get something...then once you have another job secured and ready to start, you can quit that one. Or you could just quit and go on unemployment(I mean I assume you qualify if you have been working there a long time) and search for a job on that, but I think the first option is better.
But yeah if they want to give you more responsibilities and such it would make sense they should pay more for that. So it does seem like they are kind of trying to take advantage.
I mean if my job wanted to make me a supervisor I don't think I'd do it unless they were willing to pay more, and even then...I really don't think I would be cut out for that so I'd probably have to turn that down.
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We won't go back.