"With the years, you lose a bit of intensity. Intensity in your self-belief, concentration, being positive, believing things will go well—those are in the mind. With the years you lose that a bit," he told the Spanish daily El Pais in an interview published Monday.
The 25-year-old world number two led Spain to a fifth Davis Cup title in December and won the French Open this year.
But he lost in the Wimbledon and US Open finals to Novak Djokovic and surrendered his number one spot to the Serb as a result.
"My mind was good in the first half of the year, not perfect because I needed a bit more in the games against Djokovic. But it was fine," he told the paper.
"I have started to work, fight again.. but clearly I lacked a bit in my level of tennis. When you have it, your mind responds better. I have needed to be less predictable playing. I have been too predictable many times during the season," Nadal said.
"Those are things that I have to recover for 2012."
Nadal said he had to play more inside the court to overwhelm his opponents and take control instead of letting them dictate matches.
"In 2011 there have been moments where I have lacked an extra something," he said."
The Spaniard said he had a good 2011 overall but felt he was "bad" towards the end of the year and "very bad" when he crashed out of the season-ending ATP World Tour Finals to Jo-Wilfried Tsonga.
"It was not a complete disaster but I did not win because I was too anxious," Nadal said.
"I tell myself: 'If I do a bit more I will be nearly back to winning anything again'. That is the motivation. Recovering the extra will that makes you give a bit more of yourself," he added.
"I have to get back my cruising altitude, which should be higher. From Indian Wells to Wimbledon and the London Olympic Games is where I have to recover my best level."