And last of all he was seen of me also, as of one born out of due time. For I am the least of the apostles, that am not meet to be called an apostle, because I persecuted the church of God. But by the grace of God I am what I am: and his grace which was bestowed upon me was not in vain; but I labored more abundantly than they all: yet not I, but the grace of God which was with me. (1 Corinthians 15:8-10, emphasis mine)
In his first letter to the Corinthians Paul spends quite a bit of time discussing the importance of the resurrection as there are some who are teaching there is no resurrection. Paul casts down this idea by saying if there is no resurrection then we have had faith in Christ in vain. If there is no resurrection, then Christ is not resurrected and his (and our) faith is all for naught and we are all still in our sin. He goes on to reiterate that Christ is resurrected, that He showed Himself to Cephas, the disciples, to James, to over five hundred others, and finally to Paul who was then called Saul. Paul tells the readers that he isn't worthy to be an apostle as he persecuted the church, yet Christ still called him to be one and by the grace of God he labors abundantly.
Sometimes we cat glimpses of ourselves and we realize that we are not worthy to be called after His name. But praise God it is not by our worthiness that we are called, but by Christ's! Previous failures mean nothing when we come to Christ, because now we have a strength and a guide that is outside of ourselves, truly it is not us doing the work anymore but Him through us. The past has no bearing on the future when we are in Christ. Like Paul we find a new beginning, as dark and sinful as our past was, Christ can lift us up to do great things, and all me must do is trust and obey.