Yom Kippur is the climax of the 40-day period which started in Elul.
Neilah is the climax of the day of Yom Kippur.
Neilah, as its name implies, is the closing of the gates of Heaven, the final judgment.
This is the time to reverse evil decrees.
Despite the weariness or lack of strength at the end of the day, at the end of the fast, one should arouse oneself to daven with intense concentration and emotion, and with resolve to improve in the coming year.