From what I could find, no one can pinpoint the exact source of this quotation in terms of when Shaw said it - but since it involves a sense of human potential and progress a look at the history of his spiritual life would most likely put it after he rejected atheism at age 30, when he became a "mystic" who believed we evolve as humans because of a mysterious "life force."
Why is this quotation so compelling? Because it pretty much sums up secular humanism, the prevailing outlook of our culture, based on the absence of God in our lives and the full responsibility of humans for ourselves. It is attractive, because it puts the power of who we become in our hands. It is also frightening, because if we fail, then it is simply our own fault, despite the circumstances of our life which might make "creating yourself" difficult.
However, for Christians, this outlook denies the action of the Holy Spirit, who bestows on each of us specific gifts and talents. If we take credit ourselves for all that we become, we miss that this really involves blessing - the God who knew us before we were in our mother's womb (Jer 1:5) and created us each as a uniquely gifted person. Paul recognized this when he acknowledge in Galatians 1:15 "But when God, who from my mother's womb had set me apart and called me through his grace..."
Certainly, it is up to each of us to reach the potential for which God created us - but WE certainly do not create ourselves. We cooperate with God's grace to become all we can be, using our gifts to build up the Kingdom of God in the world we live in, sometimes referred to as "co-creation"- but this is not our human work alone, nor is it equal partnership with God - rather it is submission of all that we are to the purpose of God. It is not about who we become as independently empowered individuals, but about who we already are, through our unique giftedness in the Spirit, ratified in baptism and faith in Jesus Christ. We accept the action of sanctifying grace in our lives and allow IT to "create" us. (Catechism of the Catholic Church 1999-2000). The CCC further explains how this works:
Grace is first and foremost the gift of the Spirit who justifies and sanctifies us. But grace also includes the gifts that the Spirit grants us to associate us with his work, to enable us to collaborate in the salvation of others and in the growth of the Body of Christ, the Church. There are sacramental graces, gifts proper to the different sacraments. There are furthermore special graces, also called charisms after the Greek term used by St. Paul and meaning "favor," "gratuitous gift," "benefit. Whatever their character - sometimes it is extraordinary, such as the gift of miracles or of tongues - charisms are oriented toward sanctifying grace and are intended for the common good of the Church. They are at the service of charity which builds up the Church. (CCC 2005)Certainly our purpose in the world is to become more and more holy - more like Christ, through exercise of the virtues. We can work on developing the "human virtues" of prudence, justice, fortitude and temperance, but these are the result of the "theological virtues" of faith, hope and charity, which are actually not our human work, but the work of God, in whom they originate. "They have the One and Triune God for their origin, motive and object." (CCC 1812) You cannot by your own effort, make yourself have faith, hope or charity. Your job is to be open to them as gifts offered by God for you and for others.
This Pentecost, give thanks to God for the unique gifts with which the Spirit has filled you, pray for discernment on how to use those gifts to become more who God intends you to be. You cannot "create" yourself. You can only live in cooperation with your giftedness, call and destiny. God already knows who you can become. Let go of your egocentric clinging to self-empowerment and allow God to "create" who you will become. In seeking his will, you will discover who you really are.