Cancelling my martial arts contract?

Basically i was going to this martial arts place for a few months then i thought it was a good option of i start to pay monthly rather than lessonly and also wanted to do kickboxing but they said you have to sign up to pay monthly for that, so as so i brang in my dad he signed the papers blah blah 2 months in i was rechecking the papers and his written down 36months contract. untill i can stop. So i went to speak to him thinking he had done a mistake, but then tells me thats what is signed to when i clearly dont remember siging up to this, and now the other problem is i dont find myself going because im finding my asthma is getting worse and iv had a knee injury(oscar schlatters) for a while which is getting worse, as i didnt take my doctors advice to quit any contact sports really, and im finding myself not going to as many lessons as i thought i would be going to, and at 90 pounds a month! its seeming to be pointless, cause i cant go to everylesson! nor can i cancel, so im basically giving 90 pounds to them for free now. i got to make 1 lesson in a week and finidng diffuclt. When i spoke to my instructor he didnt seem to care to much and just went you cant get out of it except if you have an injury or you move house 15 miles or more. so im not sure what to do from this step?