Authours rights for copyright under a cotract/ affidevit

I'm a new medical authour, approached by a publisher, i m writing solved paper series, as asked by them, on the basis if 12.5 persent royalty, only on sold copies and will get 5 free copies only of each edition. I started writing, provided them with 3-4 titles also, still they didnt make agreement , but finally signed a document like an Affideviton 100 rs Stamp after lots of insistance by me.it certifies joint copyright , done for three years. They got 3 titles publishedlast month But i feel the Afidevit is totally in favour of publisher My question is that can i get rid of this cotract or whatever it is and publish it by my own Presently what rights do i possess regarding copyright of the matter given by me