To begin, one must define equanimity. Equanimity is composure and mental calmness.
Victor, in Mary Shelley's Frankenstein, is only able to restore his equanimity through death. Victor's mental stability and composure is lost once his obsessions with restoring life begin. The death of his mother first instills in him a quest to bring life back to the dead. Once at school, Victor's obsession with life compounds. The closer he gets to completing his being, the more obsessed he becomes. This is where Victor's mental calmness begins to deteriorate. Victor refuses to see anyone, locks himself in his laboratory, and presses on with his scientific work (even without regard to his own health).
Once the Creature comes to life, Victor is so frightened by the being that he flees from his flat. From this point forward, the Creature is his main obsession. He fears the Creature so much, and his sole part in its creation, that he becomes violently ill. With each meeting with the Creature from that point forward, Victor always becomes deathly ill.
After his destruction of the second creature, and the warning from the Creature, Victor becomes even more obsessed with the death of the Creature. It is not until Victor's death that he can find peace. (One could even assume that Victor never finds peace (mental calmness and composure) given he dies knowing the Creature still lives.