The Irishman will be out of contract at the end of next season and knows that he will have to put in some good performances to earn a new deal.
"I have another year left on my contract, so next year is massive for me," said Thornton to the CT.
"I want to get my head down in pre-season and come back and hopefully kick on. I want to get myself really fit.
"I did well in the fitness tests we did recently but it is not the same as playing games and getting your match fitness. I didn't really feel that sharp on Monday because that is the first time I have started a game since I came back from loan at Brighton, so I don't feel as sharp as I should be.
"I would like to be involved in the last few games and if it means coming off the bench then I have got to do well again and go from there."