Before I formed thee in the belly I knew thee. Jeremiah 1:5
God knew us before we were even conceived in the womb. Not only was He aware of our identity, but He also knew every "mistake" we would make. There is no mistake too big for God to overcome. Consider Adam and Eve, God expressly forbade them from eating of the tree of the knowledge of good and evil but they disobeyed and ate of it anyway (Genesis 2:17 and Genesis 3:6). Although this was a terrible mistake, it ushered in the greatest manifestation of love the world has ever known through the death, burial, and resurrection of Jesus for us on the cross.
Conversely, there are times when we believe that we have heard the voice of God and are obedient to His promptings. Later on, we may second guess ourselves and believe that we have made a mistake when things appear to be going in the wrong direction in our lives. Remember though that "a double minded man is unstable in all his ways" James 1:8. We must not be like the Children of Israel who heard the voice of God, came out of Egypt, and later began to doubt God and the decision they had made to obey Him (Numbers 11:5). We must trust God whether we have actually made a mistake or it looks like we have made a mistake. If we have made a mistake, God forgives us of our sins (1 John 1:9) and can still make a masterpiece out of a mess because He already knew every decision we would make long before we came into existence. Alternatively, when we have heard the voice of God, we must continue to trust Him to work everything out for our good even when it looks like things are chaotic in our lives (Rom 8:28). Either way, there is no need to worry about making an insurmountable mistake!