Most mature adults don’t believe in magic.
If something is so surprising, so out of the ordinary, so unreal, then guess what? In all likelihood, it is not real. It must be a sleight of hand, an optical illusion that gives the appearance of something that it’s not.
This intuition - to reject that which flies in the face of our daily experience - can deter one from doing teshuvah, because the premise behind teshuvah does just that.
“You can’t turn back the clock” is axiomatic to our experience. There are no “backsies” in this world. Whatever was done, remains done. Yet, when it comes to teshuvah, the Torah states otherwise. When one does sincere teshuvah, he wipes out the past, as if it never happened at all. He presses the “Undo” button and restarts as if nothing ever happened.
Teshuvah works like magic - but like real magic.
Teshuvah was created before the world, before the very creation of “time,” and therefore is not bound to “before” and “after.” Just as the future can be changed so can the past.
The pasuk therefore states regarding teshuvah: Hashem! Return us to You, and we will return. Renew our days to how they used to be.
This idea is amplified also in the pasuk that states: Create for me a pure heart, Hashem, and renew a spirit within me.
If you truly accept that teshuvah works, you can look at yourself in the mirror and see a new, cleansed person. You need not be saddled by guilt, because the stain is no longer there.
In fact, the Rambam states that as part of the teshuvah process, the person has to dissociate himself from his sins. “That was not me. I am no longer the person who committed those sins.” And by doing so, by letting go and distancing himself from his past, by denying that it is part of who is he today, Hashem grants him his wish. Through the magic of teshuvah, one is allowed to hang a “Grand Re-Opening” sign, to restart himself, to shed his past like a worn coat.
Of course, there is a process to teshuvah, and it involves reflecting on the past and regretting what one has done. But the sefarim emphasize that these thoughts should be relegated for specific times when a person “does” teshuvah; they should not constantly weigh him down. By following the guidelines of teshuvah, the past is in fact erased.
In a fascinating responsa, the Noda BiYehudah was asked to set forth a program of teshuvah for someone who had committed multiple heinous sins over a three-year period - sins which would have been punishable by death at the time of the Sanhedrin. It is amazing to read the respectful tone he employs when addressing this individual. It is so abundantly evident from the way he writes to and about this individual, that to the Noda BiYehudah there was absolutely no reason to see this individual as somehow tainted and besmirched. On the contrary, he is to be celebrated and respected for doing teshuvah.
Teshuvah really works!