The former Wigan back has two-years left on his contract with the West Yorkshire outfit but was told he would not feature at The Jungle in the upcoming campaign.
Mathers has decided to move away from the Tigers on a year-long loan deal just down the road with the Wakefield Wildcats as he looks to get some regular rugby under his belt.
The 28-year-old is now looking for a fresh start at Belle Vue but has admitted he does not know what happened with his time at Castleford.
He said: "I felt like an outcast. It's a lonely old place. You spend a lot of time on your own and it was a good job I've got a good family around me, people I could talk to and let my emotions out.
"It's quite refreshing to know that you are wanted (at Wakefield) because you do doubt yourself.
"I just want to enjoy this year and we'll see what happens."