On the day that one decides to repent, a person should cast away all their sins and consider themselves as a newborn – having neither merit nor sin. a person should live today as if it is the beginning of one’s deeds. From this day onwards examine ones ways so that one should not depart from the path of good.
A person should not be deterred by thoughts of doubt (that maybe one is incapable of penitence) that undermine the desire to repent. Instead consider how “good and upright Hashem is, He guides sinners on the way” (Tehillim 25:8). Therefore, a person should cast off their old ways and make for themselves a new heart.
Rabbeinu Yonah in his Yesod HaTeshuvah