In the 5th bracha of the Amidah, we request from HaShem "our father" (אבינו), to return us (השיבנו) we mention three kinds of Teshuvah – 1. returning to HaShem, 2.returning to the Torah, and 3. returning in “complete” Teshuvah.
The first two I get, but the understanding of the last kind of Teshuvah I don't get.
Rabbi Mordechai Becher that this kind of Teshuvah is a return to the purity of one’s soul:
אלוקי נשמה שנתת בי טהורה היא, אתה בראתה, אתה יצרתה, אתה נפחתה בי
... "My G-d, the soul you have given me is pure. You created it, You formed it, You breathed it into me ...": Teshuvah, literally translated means “return.” We believe that the soul is intrinsically pure and began its sojourn in this world in a state of purity. Mistakes and wrongdoings are departures from the essential nature of the human soul. Therefore when a person has done something wrong, the process of teshuvah is really that of going back to his or her true essence.
So Teshuvah doesn't only mean to return to HaShem and His Torah, but it also means to return, to return to the original state (its pristine core; to one's original nature) we were in.
But how does one achieve this? How does one return to his original self; what does this even mean? And how does this helps a person not to do a sin anymore in the future; wasn't it from this state of being that one sinned in the first place?