Do you know why as Christians grow the bolder they become? It's because they become more and more rooted in Christ. Every fruit bearing Christian has enough root but the question is are the roots becoming deeper and deeper? Are the branches pruned whenever they're needed? Remember the parable says that there's some that bore more fruit and others less fruit and these are all believers. As Christians have varying degrees of fruit bearing and maturity levels, the only way to be more mature is to have an ever constant intimacy with God.
Failures come when Christians rely on themselves instead of the grace of God to conquer sin. Before Peter denied Jesus, he had the problem of pride. It was the "I" problem and it's the middle letter of PRIDE. Only when Peter learned to humble himself did he really become stronger after that experience. Considering that the Christian life has never been you're saved and you're on your own then always ask for God's grace because it's the only power against sin they have. The Christian has no power against sin apart from the saving grace of God. On their own they fall but with God's grace they stand.