It does not matter where you have been, whom you have been, when you are in Christ you are a new creature and God can use you for His glory. The biggest sermon you can preach is telling people what God has done for you. God is very particular, He wants things done the way He has instructed.
Never assume that the conquered sins of yesterday have no power to put you down. There are moments in this life the God you love and the God who loves you will have to say no to what you really want, but you need to remember it is a no from a loving Father. When God says no, It is grace in action. If God allows disappointment in your life, wait upon Him, he is preparing something better.