Too quick to sell copyright

I spent two years developing bookkeeping and payroll courses for a distance learning college (on a contractual basis). Unfortunately I was really green at the time and didn't understand what I was giving up when I signed a contract to give them copyrights to my work. I had been receiving regular payments for tutoring the students via email and telephone but, when the work was completed, they decided that instead of paying me per course they would pay me per student and that meant I would receive only a third of my original payments but with no reduction to the work involved. As it was I was working 12 -16 hours a day, 7 days a week and I didn't have a single holiday in that 2 - 2.5 year period! Obviously this situation was unacceptable to me and I did not sign the new contract. So I have now lost all income from my work and have had to start over with a new college. I know I was green and I shouldn't have sold the copyrights in the first place but, given that I sold the copyrights at a ridiculously low price to begin with (in order to secure the business) is there anything at all that I can do to salvage something from my 2 years of hard slog? The college currently make around £1,200,000 per annum from my work alone and they deal with writers all the time so they were well aware that the payments they made for my work were extremely low (£350 per manual for 10 manuals). Can anyone give me some advice on whether or not I can do anything about this?