Jesus was fully in harmony with - aligned-with - God's creative purposes; but we mortal men are not.
We may partly - sometimes, but briefly, wholly - be aligned with God's creative purposes; but nobody (except Jesus) has always and in every respect been in harmony with creation.
Therefore all Men are sinners. And Men can do nothing about this, because in mortal life we are all flawed and there is always change (and the possibility of change); there is disease, corruption, decay - and death is inevitable.
Sin is not being aligned with creation, not being in harmony with God's will.
By analogy sin is being turned away from God's creative purpose; turned in some other direction than where God wants to go.
Repentance can mean 'turning' - so repentance is to turn and face in the direction that God wants things to go; it means to be in harmony with divine creation.
Jesus saved us from sin because he made it possible for us to be aligned with God's will fully and forever - but on the other side of death; by any of us who so chooses to follow Jesus through death to eternal and divine life. That is to be saved from sin.
Meanwhile, this mortal life is intended as a time of experiencing and learning - which is why it is subject to disease, corruption and decay; and why it is not possible to be always and wholly aligned with God's creative purposes.
Note: What people usually call The Sins are, properly, actions (including thoughts) that are observable consequences of the real sin, which is inner and motivational - which is the state of being not-aligned with (including opposed-to) divine will.