Perhaps it was impatience, mistrust or even despair. Whatever the reason, you thought you had a better way. His way may seem higher, harder and completely unsafe, but it is always ultimately good.
Take a rest on your journey to read these passages and ponder this question:
Genesis 16; 17:18-21
In what ways have you attempted to take God's promise into your own hands, and how has His grace been shown despite your lack of faith?