Often we get it mixed up. We tend to believe we are not what God wants us to be based on our performance, in our walk with Him. We don't seem to measure up to what we assume God's standard for our existence is. Yes, God has a standard of performance for those who have committed to following after Him. It is the Word of God. But His Word tells us who and what we are, not based on performance, but on His infinite grace. "But by the grace of God I am what I am, and His grace toward me did not prove vain; but I labored even more than all of them, yet not I, but the grace of God with me." (1 Corinthians 15: 10)
If we understand first who and what we are, and that who and what we are is determined by God (not us), then what we do for Him will be done for Him with our eyes on Him, not on ourselves. Grace is what made Paul who and what He was, in spite of His horrible past. Paul realized that even the labor he performed was not Him, but God's grace with Him. In other words, he couldn't do what he did without God's grace extended to him. Understanding this empowered Paul to keep His eyes on Jesus, even when his past was thrown up to him by the devil, people, and even himself.
Grace is unmerited favor. Grace cannot be earned. Grace is imparted by a loving, forgiving God. Grace is powered by God's infinite love for you, me, and all of mankind.
In order to fully understand Paul's statement in the above verse, we must understand his circumstances. "For I am the least of the apostles, and not fit to be called an apostle, because I persecuted the church of God." (1 Corinthians 15: 9). Paul struggled with the fact that he once persecuted and ravaged the very church he now served. He was a pretty bad dude to the early church, dragging folks from their homes, putting some in jail, and more than likely, putting some to death. Paul stood by approving the stoning of Christianity's first martyr, Stephen. It would be impossible for him not to regret what he had done.
Paul knew what he had done. But listen carefully. Paul also knew what Jesus has done! You and I obviously know what we have done in our pasts. We know what we struggle with today. And like Paul, we must know and understand what Jesus has done for us. He took our sins upon Himself and died to them. The second our relationship with Him began, we died to all of our sins. He did it, not you and me. Also, the second we began our walk with Jesus, we began to live for Him. Because Jesus didn't just die to our sins and stay in the grave. No, three days later He rose out of the grave in supernatural, resurrection power. So now we live with Him, in victory over our past and present struggles! He did it. Grace. Because of grace, your past and your present will never ever determine who and what you are in God's eyes. His grace is more than enough, and by His grace you are what you are!
When people understand this, it gives them the power to overcome their struggles. Listen, you'll never be perfect in your own eyes. But it's not up to you to determine who and what you are. You'll never be perfect in other's eyes. It's not up to them either. By the grace of God, you are what you are. Nothing will ever change that, not even your past or present short-comings.
Perhaps you feel you've thwarted God's plan for your life because of your past. Perhaps you feel God is done with you because you struggle today with things from your past. Know that "...the gifts and calling of God irrevocable." (Romans 11: 29) How could this be? Because it's not by you, but by the grace of God that you are what your are. And because, "...if anyone is in Christ, he is a new creature; the old things passed away; behold, new things have come." (2 Corinthians 5: 17) This is grace. All God. Zero you and me.
And finally, if by God's grace, you are what you are, you need to start calling yourself what you really are: a man of God, a woman of God, a child of God, a new creation in Christ, etc. Thank God continuously for who and what He has made you to be. When the enemy comes to you and begins accusing you of who you were and what you did, remind him that by the grace of God, you ARE what you ARE. That's who you really are, and that's what really matters.