"It's been hard," Defoe told the Daily Mail.
"You start to think: 'What am I doing here?' I'm used to playing in big games. I've played in big games for my clubs and for my country. It'll be a tough game. I hope I'm playing and if I can score and we win the game then it'll be great. But I remember playing against Charlton and scoring two goals (in a 3-0 FA Cup win in January), then the next weekend I missed out.
"I was upset about that because once I start scoring I'm confident and I have the feeling I know I'll score in the next game. We played Manchester United and I thought, 'I'm playing', but I didn't start."
Asked whether he considered following Robbie Keane and going on loan after the Irishman joined West Ham in January, Defoe said: "I don't think the club would have allowed me to go on loan. But it was like when I decided to leave here and went to Portsmouth because I got to a situation where I thought, 'I'm not sitting on the bench'.
"I'm 28, I don't need to be sitting on the bench. I don't need anyone to put their arm round me and say, 'Just keep it going'.
"And if it means I have to work even harder, then I will. I'll stay back after training, I'll work hard, I'll do running. Whatever it takes. I'll do everything right off the pitch.
"I've just got to keep working hard, keeping sharp and making sure I'm ready when I do get the opportunity."