Our Success Is in God
Matthew 26:33 "Peter answered and said unto him, Though all men shall be offended because of thee, yet will I never be offended."
Just like Peter, we are often more confident of ourselves than God is. Many Christians are devastated when they fail in some area that they thought they had long since grown past. We need to realize that as long as we are in this mortal body, we cannot fall asleep at our battle station against the works of the flesh. If we do, our flesh is just as capable of sin as anyone else's. If we fall, it should concern us but not surprise us. We should simply realize with Paul, that "in my flesh dwelleth no good thing," get back into the Spirit through confession and forgiveness, and go on with Jesus.
The Lord has never had anyone who was qualified working for Him. Peter wasn't qualified, and even at our best, neither are we. If we realize that we are nothing and have nothing, then we are prime candidates to be used by God.
Peter went on to become probably the best known of the twelve apostles. The Lord used him mightily. But He didn't want him, or us, to forget that it was Christ in Peter, and not Peter himself, Who was great.
This also illustrates the extent of our Father's love and forgiveness. If God not only forgave Peter for his sin, but also reinstated and advanced him, then surely we have not pushed God's grace beyond its limit. Where sin abounds, grace abounds much more.
God doesn't see us as a failure - just a learner. God can redeem the worst "failure" and work it together for good (Romans 8:28). We need to think like God.