The 25-year-old midfielder was club captain last year but was frozen out by Di Canio and, he says, made to train on his own or with the academy players.
"I was very close to leaving a couple of times," he said ahead of Sunday's clash with Newcastle. "For whatever reason, it didn't happen. The way it was going, I would have left happily. I wasn't part of the manager's plans and that can happen in football.
"Yes, it was torturous, but I've always been quite confident and I've got a lot of experience of managers who have backed me. I didn't lose confidence over anything he did or the fact he didn't fancy me. I worked hard with the hope of getting a chance."
Cattermole was full of praise for Kevin Ball, who restored the midfielder to the starting XI when he took over as caretaker manager, but admitted he was unhappy with the way he was treated by the club.
"Things happened in the summer and there were a lot of things I was disappointed with from the club in terms of the way I was treated," he said. "I understand that is football but there are ways of doing things.
"All you ever want is a bit of respect and Kevin Ball gave me that straight away.
"I am more mature now, 100 per cent. The summer was a massive learning curve for me. It is the first time I have not been wanted by a manager.
"I turned round and I was quite happy with how I dealt with it. I spoke to a lot of senior players who have been in the same situation. I listened and took advice. I kept my mouth shut. I kept my head down. The only thing I could affect was what happened if I got my chance. I kept training each day. A lot of the time I was running on my own."
While the dressing room appears to be a happier place under new boss Gus Poyet, results have not improved.
Cattermole knows how important it is he and his team-mates put that right against north-east rivals Newcastle on Sunday.
"We have chatted about the importance of the derby," he said. "The manager has made it clear to everyone how important it is - as if people didn't already know. Come Sunday, when they arrive, they will certainly know."