Confidence is the memory of past success.
As I get older, I do become more confident within myself. It's as if my life has been a giant preparation ground for some big event in the future. However, I have always loved the quote "Life is not a dress rehearsal" too. So the concept of being in preparation for something ahead would not be so true in that case. We are always "ready" for what life throws at us in the here and now. God never gives us more than He knows we can handle.
Life is. Unless you happen to believe in the reincarnation of the soul into other life forms, then, for those of us who don't... this is the ONE life we have to live for all eternity. A consistent and continual moving forward and an experience rooted in the self growing ever more confident and wise.
Growth in living our daily lives is the result of both mistakes and successes. Growing older can be fraught with all manner of physical and psychological ailments but growing wiser and more confident is gift of inestimable value.
We need both confidence and humility in balance. The memory of past successes helps us to help others succeed too. We've been there done that etc. But we also need to know the memory of our past mistakes to keep us humble and not too assured of ourselves. Confidence may be the result of knowing past successes but Humility and the resulting wisdom that springs from that is the result of knowing past failures too. Both are two sides of Life's coin and both are needed for a person to become beautifully whole.