The Rambam writes that any person who desires perfection can reach a level similar to that of Moshe Rabbeinu, and he who desires evil can reach a level similar to that of Yeravam ben Nevat. Rav Wolbe (Alei Shur vol. II pg. 40) explains that the decision to go one way or the other can be decided in a few minutes, however, the actual process must span many years. It is not in a person's ability to change from a rasha to a tzaddik in one day. Although every person has bechira, it is basically impossible to exercise his bechira to the degree that will allow him to turn around 180 degrees in one moment.
The Mashgiach quotes Rav Dessler who ingeniously explains the concept of bechira. When two countries wage war, the objective of each country is to conquer the entire territory that is presently under the dominion of the rival country. However, the actual battle takes place on only one front. When the battle is over, one country has added territory while one has lost part of its territory - and the battle then moves to a different location. The war between the yetzer hatov and yetzer hara is no different. Most of our actions are not a product of our bechira because they are not at the battlefront. There are many mitzvos and good deeds that a person does without choosing at all; rather, they are dictated by the way he was brought up or his intrinsic nature. Likewise, there are many aveiros that he does without even realizing that they are wrong; it is simply the way he was educated. His nekudas habechira (point of bechira) is merely at one specific location: where what he knows to be true clashes with what he imagines is true (but deep down really knows it's not). For example, many people speak lashon hara without even realizing that there is anything wrong with what they are doing. However, the yetzer hara will not come to these people and try to convince them to be mechaleil Shabbos. Since they were little children they have been habituated to keep Shabbos and the yetzer hara has no chance of gaining a foothold in this area.
We must approach Yom Kippur with this idea in mind. There might be many aspects of Torah and mitzvos in which we feel that we are deficient. However, our teshuva must begin with the aveiros that are within our nekudas habechira. One who on a daily basis davens without any concentration cannot change this pattern in one moment. He must try to rectify this problem gradually. If he feels that it is in his ability to concentrate while he recites "Birkas Hatorah" then he should start with that. This is why Rav Yisroel Salanter said one should make a small, practical but ironclad kabbalah (resolution) before Rosh Hashana. After finding one's nekudas habechira, a kabbalah will help him condition himself in a small aspect of his avodas Hashem, thereby conquering territory from the yetzer hara. He will have won this battle and moved closer toward winning the whole war.