Teshuvah is hard to define because of its complexity.
To understand its essence, we first need to contrast it with the concept of law.
A "law" is something we cannot transgress without causing or experiencing the consequences.
This is obvious regarding, for example, the laws of nature: I cannot put my hand in a flame without getting burned or walk barefoot in the snow without getting chilled.
So it is for the laws of the spirit.
Worlds other than the physical world are also governed by laws.
I cannot sin without it having an impact both on my soul and on the world as a whole.
The entire universe, both physical and metaphysical, is profoundly affected by causality.
All of our actions have effects.
These effects are generally irreversible.
I can jump off a roof and then exclaim, 'That's not what I wanted to do, I'm sorry I did it, let bygones be bygones . . ."
Obviously, I may be sorry I jumped off, but that does not make any difference.
What is done is done.
What is true for matter is true for the spirit and ethics as well, because they, too, are governed by causality.
If I commit a sin, if I transgress a Divine prohibition, if I neglect a Divine duty, what is done is done, and what has not been done has not been done.
My actions, or lack of them, change reality.
Teshuvah runs counter to these starkly obvious examples and irrefutable logic.
Teshuvah consists of saying to God, "I didn't want that to happen," or " I am sorry I did that," and then. to obtain Divine forgiveness.
To behave as though these events never occurred.
How can something that took place, that exists, be no more?
This is the crux of the problem.
The first step in teshuvah does not consist of standing before God and hearing Him say, "I have forgiven."
Before this happens, I need to tell Him about my life, my past, my world, and ask Him for something much more radical than forgiveness.
I want my sin to be blotted out.
It's much more than changing the future.
It is changing the past, so that yesterday is no longer yesterday.
In a world governed by causality, this cannot take place.
It can only happen when we come before God.
–Rabbi Adin Steinsaltz
From The Seven Lights, p. 55 by Rabbi Adin Steinsaltz