The Catalan midfielder returned to Barca this week to conclude one of the most protracted transfer sagas in world football.
Barca paid an initial 29million euros to sign their youth team product from the Gunners after he made it clear he wanted to return to the Nou Camp.
Fabregas has spoken of his delight at completing the move but admits he faces the biggest challenge of his career now.
"The easiest thing would have been to stay at Arsenal," he told Barca TV.
"I'm the captain, playing every game. We are always in the Champions League, always fighting for titles.
"For all that, and the tough competition here, perhaps now might be the worst time to return to Barca. It's going to be really tough to make the starting XI, but I needed a new challenge in my life.
"I thought I'd stagnated a bit, and needed a change of scenery. I needed a special motivation to bring the best out of myself."
Fabregas will fight for a midfield place with Xavi and Andres Iniesta, as well as youngster Thiago Alcantara.
His signing is viewed as a long-term acquisition with a view to him taking over from Xavi in the future but Fabregas played down the suggestion.
"I don't know," he insisted.
"I think that Xavi still has some years left in him! He does things you wouldn't believe!
"I haven't come to retire Xavi. I came here to play with Xavi, to compete with Xavi, and to win with Xavi.
"What I'm saying about Xavi also goes for Andres and Thiago. I think altogether we make a very good group. I know what we'll do. With the great group we are, we'll do great things together."
The 24-year-old is simply relieved the saga is over after spending much of the summer in limbo hoping the move would happen.
"The last months I've been under a lot of pressure," he admitted.
"Mentally it's really tiring, but it finally happened and I'm very happy to be back at Barca.
"It is partly my fault that it has been so complicated, for having left. Not in my wildest dreams had I imagined that one day I would play for the Barca first team.
"Not as a child, nor playing with Arsenal had I thought I'd live a day like Monday."