"Sin" or "Chet" in Hebrew, means "failure in our relationship with HaShem (G-d)."
Our goal should be to continually move closer to G-d, But "chet" is behavior which causes us to move away from G-d.
"Repentance" or "Teshuvah" in Hebrew, means "Return."
In this context, it means "Return to G-d" and to behavior required of us by G-d.
While we deeply regret our movement away from G-d, we should not despair. Rather, we should bear in mind the verse written by King Solomon, the "wisest of men," in the Book of Kohelet (7,20),
"There is no one
So righteous in the world,
Who does only good,
And does not sin."