Having recently watched FC for the first time in a while, I think that too much is made of Picard being "out of character" for the sake of the film.
First, Picard's experience with the Borg was supposed to be like a rape...he might have learned to deal with it, but he wouldn't have just gotten over it.
Second, the stakes were much higher and more personal in the film than they had been in either of the Borg's follow-up appearances on the show. They were returning to Earth, they were assimilating his own ship (a figurative extension of himself), and threatened to undo everything that he held dear (the "origin" of the Star Trek setting).
Third, it was a slow burn, he didn't just start acting crazy first thing that they appeared, he got more intense as the stakes got higher.
Fourth, it was a nice twist on TWOK that this film had the hero in the Ahab role.
Fifth, with everything that was at stake in this situation, he may have been angrily kicking himself on the inside for having chosen not to take proactive measures against the Borg back in "I, Borg".