Confidence is a good thing, so long as we get our confidence from the right source that is God Himself. The Bible has shown that time and again, man has tried to reach great lengths on its own, and fall into the sin of pride. The tower of Babel (see Genesis 11); the census of Israel conducted during David's time (see 2 Samuel 24; 1 Chronicles 21); and the "voice of a god" speech delivery by king Herod (see Acts 12:20-23) are merely examples of such self-confidence and consequent defiance of God.
As a Christian, we must remember that being confident so that we can do great things is good, but we need to put our confidence in the Lord, who has designed us and continually enables us to do good works. There is no other way. Paul said in Philippians 3:3 that those who worship God "rely on what Christ Jesus has done," and "put no confidence in human effort."
But I was the one who worked on it!
Sure, you can say that it was by your effort that such and such a thing was accomplished. Good for you! But think about it: who gave you your breath? Who gave you life? Who has designed you and has given you your potential but He who made you carefully, even in your mother's womb. Friend, we can only do the things that we are doing because God has willed that we could!
The Bible says that "There is none righteous, no, not one; There is none who understands; There is none who seeks after God. They have all turned aside; They have together become unprofitable; There is none who does good, no, not one." (Romans 3:10-18; Ecclesiastes 7:20).
Friend, while God recognizes our skills and capacity, He also recognizes our inability to do some things. These are some reasons why we should put our confidence in God alone:
1) We cannot work for our salvation
Ephesians 2:9 tells us, "Salvation is not a reward for the good things we have done, so none of us can boast about it." No one could ever work for their salvation. It's not some sort of reward for being good, or some commodity that we can buy with our hard-earned cash. It's something that we can only receive by turning to Jesus Christ.
2) We can't free ourselves from sin
We can never break free from sin and condemnation with our own strength. The Bible says that "The law of Moses was unable to save us because of the weakness of our sinful nature. So God did what the law could not do. He sent his own Son in a body like the bodies we sinners have. And in that body God declared an end to sin's control over us by giving his Son as a sacrifice for our sins." (see Romans 8:1-3)
3) We can't have a relationship with God Himself in our own ways
We could never have a relationship with God on our own accord. But thanks to Christ, "now all of us can come to the Father through the same Holy Spirit because of what Christ has done for us." (Ephesians 2:18)