The Argentina forward, who levelled Pele's one-club scoring record at the weekend with his 643rd Barca goal, has revealed his failure to force a move away from Camp Nou affected his form at the start of the season.
However, with five goals in his last seven league games, he says he's moved on from that disappointment and is ready to help Barca compete on all fronts this season.
"Honestly, I am fine now," Messi told La Sexta in an interview that will air in Spain on Sunday.
"It's true I had a bad time in the summer. It came from everything that happened before the summer, how the season ended, the burofax, all of that. I carried that into the start of the season a little bit.
"Right now, I feel good, motivated to seriously fight for everything we have ahead of us. I feel excited.
"I know the club is going through a complicated moment -- both the club and the team -- and that makes everything surrounding Barcelona difficult, but I am motivated."
Messi's contract expires at the end of the season and his future remains unknown beyond then. He is free to talk to other clubs and negotiate a pre-contract agreement from Jan. 1.
Barca, meanwhile, are waiting to elect a new president on Jan. 24 before opening contract talks with Messi.
Joan Laporta and Victor Font are two of the favourites and both have told ESPN they're confident they can sell a winning project to Messi and convince him to stay.
Laporta, Barca president between 2003 and 2010, slammed the club for "lying repeatedly" to Messi in the past but said his relationship of trust with the forward from his first spell at the club will help him persuade him to renew his deal.
Font, meanwhile, had said he can make Messi stay if he's elected and that Xavi Hernandez will play a key role in getting him to sign new terms.
Barca are currently fifth in La Liga, eight points behind leaders Atletico Madrid, after dropping points in seven of their 13 league games. They are into the round of 16 of the Champions League, where they will meet PSG and have the Spanish Super Cup and Copa del Rey to come in the new year.
In 17 appearances in all competitions this season, Messi has scored nine goals, setting up two more.