Canseira translates to "weariness" or "exhaustion." It's a less common word than its synonyms, making it stand out.
In the context of the song, after a series of apologies and reflections on unrequited love and the futility of forcing affection, the singer concludes with this single, poignant word. It encapsulates the emotional fatigue and resignation he feels from the entire situation, making it a powerful and memorable closing statement.
"Perdoa" is Joao Gomes asking forgiveness, not for doing wrong, but for having chosen to love someone and believed in a dream that didn't hold up. He looks back at everything he gave and prepared for, and admits he can't keep carrying it.
In the chorus he lets go: he won't force anyone to want him back, and leaves the outcome to time. "Quem sabe" (who knows) runs through the song like a shrug, less a plea to win her back than a quiet acceptance that some things aren't his to decide.