could you write your mum a letter? that way theres no room for an argument and you can get your feelings heard. It probably wont change your circunstances but it might help you if you can get your mum to know how you are feeling without having a row.
Divorce is difficult at times, I know that, try remember its difficult for your mum too.
I dont know if it is possible for you to still see your step dad because I do not have the information. I do think it is important for you to find someone close by that you can talk to though.Dont bottle it all inside, please find someone to talk to.