Defoe said: "It's been a relief to join Pompey. Obviously there's been a lot of speculation about my future for a while but as soon as I got the call from Harry Redknapp I knew what I wanted to do.
"It was frustrating at Spurs. Playing one game, then missing three, then playing again.
"As a footballer, I can't handle just sitting on the bench. I want to be scoring goals.
"I'm at my happiest when I'm out there so it was hard to take. Towards the end I was telling myself, ‘What else can I do?' I was coming into the team and scoring but then being dropped. If you get an opportunity and feel like you've taken it, only to be dropped, then it's hard. At times you think you're working hard for no reason.
"But you have to keep professional. I think I've learnt how to bite my tongue. There's no point moaning. If you let it affect you then, when you do play, it could mess your head up.
"Now I'm at a club with a fantastic manager and players, so I'm delighted. Now I can kickstart my career."
But Defoe, 25, insists he has no axe to grind with Ramos and is just concentrating on getting into Fabio Capello's England plans.
He said: "My relationship with Ramos was fine. At the end of the day, he's a good manager and will do well for the club. It wasn't a surprise to be left out by England. Had I been in the squad I'd have been surprised.
"But I'd like to get back into the England squad. It's a case of getting fit and scoring goals to help Pompey into Europe.
"If I do that then, hopefully, I'll earn another opportunity."
Pompey chief Harry Redknapp admitted: "Jermain was always a player I liked but never seriously thought would become available, so we're delighted he's joined us."