Can I sue Sallie Mae?

After more than two years of consecutive, on-time payments I am attempting to remove a cosigner from my loan as not to impede her buying power in the future. I have spoke with 3 supervisors at Sallie Mae who have assured me that if I take a couple steps, that my cosigner would be released. Which I gladly did (one of which was a credit card payment that was ran for a different amount then authorized). Today I received a letter stating that my cosigner would not be released because of a late payment made in June and that I would now have to make 24 more consecutive payments to possibly release my cosigner (although nothing of this was ever mentioned in the prior interactions with Sallie Mae). The last supervisor I've spoken with (the 4th on this issue) said there is nothing else I can do.