Repentance Defined

Repentance unto salvation is a turning, but not a turning from sin per se. Repentance unto salvation is turning from your self-righteousness and self-justification to come to Christ, confessing that you are a sinner deserving judgment. It is ceasing to believe in the worth of yourself, your goodness, your good works, or your religious activities, and turning instead to trust only in what Christ has done on your behalf to save you. It is acknowledging that faith in Christ alone is really the only thing you have that you can offer to Christ, and the only thing from you He would accept.
It is not turning from sins to receive forgiveness for your past. It is admitting your utter inability to do so, and trusting God to be as merciful as He says He is, so that He will forgive you, even though in your own strength you absolutely cannot turn yourself away from your sins or your sinful nature to follow Him.
And, of course, the only basis upon which God can grant that unearned forgiveness, is that Christ actually did earn it for you, by living a perfectly sinless life in your place, and then dying in your place on the cross, absorbing the punishment you earned, and rising from the grave alive again to save you forever.