We are all human, we all make mistakes and God can still use us to fulfill His glory. David brings up a terrific point in his Psalm -- God does not only use us in His plan, but our imperfections actually enhance His glory. God can complete miracles on earth despite our flaws. Our short comings and even our unwillingness are not able to disrupt God's plan. He uses our defiance as a weapon to make what He has already laid out even more powerful. There is a moment where it can only be God at work so all the glory goes to Him.
Help me to keep you in my focus and stay in your will. However, if I falter I pray you still use me to fulfill your plans and to give you the glory.