No matter how shamefully is our past, God can still forgive if we return back to Him with a heart of repentance. What God need from us is confession of our Sins and also turning away from it after we have repented. God can still use us for His glory because He uses Paul of Tarsus despite all what he did persecuting the Christians. Let us remember that He gave us His Son (Jesus) who lay His life down for us so that we can have a reconciliation back with the Father by shedding His blood for us.