Although the Brazilian had previously insisted that he had been able to shut out the pressure caused by Ferrari's doubts over his form, he now accepts it affected his focus.
"In the first half of the year, I was worried about renewing my contract," Massa said at Ferrari's World Finals event at Valencia on Sunday.
"I did not think that Ferrari would really wait right to the end of the summer before deciding on the driver line-up for next year.
"It's true that ever since I've been at Maranello, there have always been rumours about me. Already in my first year there was a list of drivers who were supposed to take my place and it was the same this year, with a lot of names and plenty of speculation.
"At the start, I paid too much attention to these things, but then, in August, I told myself I should only think about racing and having fun and so I began to really drive, to have the right feeling with the car and to drive as quickly as I know how."
Ferrari eventually decided to renew Massa's contract in October.
He enjoyed a stronger end to the year, scoring two podium finishes and outqualifying title-chasing team-mate Fernando Alonso at the last two grands prix.
"Now I feel very strong and the results were there to see in the last nine races, so I am optimistic for the future - mine and Ferrari's," Massa added.
- Sports & Recreation