The England striker has dropped out of Harry Redknapp's first-choice XI this season as he has struggled for goals, with only four in the league all campaign.
"I want to help the club achieve things, win trophies and get in the Champions League, but when you feel like you are working hard, training hard and being professional, and then for some reason you are not playing, it is difficult," Defoe told Sky Sports News.
"It is frustrating. At the end of the day I am ambitious. I want to play in every game and score goals, especially when you score goals for a fantastic club like Tottenham.
"I have always been like that from the first day I walked through the door and I truly believe that if I am playing games I will score goals. I think my record shows that.
"It has been really, really hard to be honest.
"Players come and go, that is just football and it is out of my hands."
He added: "I love the club and I still get the same buzz that I did when I first walked out at White Hart Lane, which is important.
"But for me the main thing is playing football and if I am not playing football then I am not happy, because I love my football."