I have been reading D Martyn Lloyd Jones' classic, Spiritual Depression. In the second chapter, he wrote,
Would you like to be rid of this spiritual depression? The first thing you have to do is to say farewell once and for ever to your past. Realize that it has been covered and blotted out in Christ. Never look back at your sins again. Say: "It is finished, it is covered by the blood of Christ." That is your first step. Take that and finish with yourself and all this talk about goodness, and look to the Lord Jesus Christ. It is only then that true happiness and joy are possible for you. What you need is not to make resolutions to live a better life, to start fasting and sweating and praying. No! you just begin to say:
I rest my faith on Him alone
Who died for my transgressions to atone.
Take that first step and you will find that immediately you will begin to experience a joy and a release that you have never known in your life before. "Therefore we conclude that a man is justified by faith without the deeds of the Law." Blessed be the Name of God for such a wondrous salvation for desperate sinners.