Good words I just heard from John Piper, "the only sins against which we have any hope of victory are sins which have already been forgiven"
Believers in Jesus Christ work against sin not so that we will be forgiven but in the aftermath of having been forgiven.
Only a believer who has been justified by faith--meaning that his sins, past, present and future have been forgiven and his sinfulness covered by Christ's righteousness--has the Holy Spirit dwelling in him.
And only one empowered by grace through the Holy Spirit can successfully do battle against sin.
Which means, as Piper says far more succinctly, every sin against which you struggle is a sin that has already been forgiven.
What an encouragement for believers struggling against addiction or besetting sin. You are not earning anything--God has already given you all things--you are rather seeking to please and glorify the one who already loves you and calls you by his name.