Imagine the most beautiful waterfall you've ever seen. Now, in your mind, stand at its base and hold a thimble under the crashing water. This illustrates the measure of God's grace in your life and covering your sin. You can no more contain a waterfall in a thimble than you can God's love and forgiveness on you personally.
If you have accepted Christ as your Savior, then you have accepted His atoning work on the Cross. God set no limits on the effects of Jesus' sacrifice, so why should you? If you are wracked with guilt over sin, whether it happened long ago or is ongoing, and you can't seem to escape its snare, remember you cannot out-sin Gods grace.
While God will judge sin and will discipline a wayward believer, He also will forgive you any transgression. And if you do it again tomorrow, He'll forgive you. this is not a license to sin but an invitation to accept God's grace humbly and repent into an obedient walk with Jesus.
The believer's goal should be to greet God's grace and forgiveness with genuine thanksgiving and contrition. We could never produce one achievement that would warrant our Lord's grace, and we could never produce enough failures for Him to take it away.
When you hold that thimble under Christ's Cross and catch one drop of His blood, you have enough.