When something Jesus said in the Bible directly parallels something in your life it becomes that much more potent. It starts to live and breathe in your heart. You can read and study the scriptures and learn lots of great things and take those truths to heart; but when you've returned from a time of grief, of spiritual dryness or even acting out your own little version of the Prodigal Son, the things Jesus said are all of a sudden said to you personally. His words are connected directly to your own experience, because Jesus went through grief and temptation like us and God knows our hearts and histories inside and out. It's similar to hearing someone talk about having been to China, in that if you had been to China yourself the speaker's words mean much more to you than if you hadn't been to China. After times of wandering away from Him (the metaphorical "trip to China") God will use you and your wandering to glorify Him in ways you wouldn't think of; taking your negativity and rejection and building something else, some positive, out of it.
There are many things in both the old and new testaments that show how much God uses broken people: in the old testament Abram said he wasn't married to his wife to avoid trouble, Moses killed a man and ran away from the consequences, David slept with a man's wife then had the husband killed, and Sampson fell famously for Delilah who did him in, among others. My one hitch in using those biblical figures is that they were all "famous Men of God" before their respective trip-ups, which is not a major point but can keep some people from taking that example to heart. After all, sin is sin is sin, no matter how legendary you are in your faith. In fact, that point may even make these people better examples, because of how far of a fall it was for them. In the end, they all made their peace with God and went back to him.
And that brings us to Paul; formerly known as Saul, the man in charge of persecuting Christians. His job was to persecute followers of Christ; he had even been part of the stoning and killing of Stephen, one of Jesus's apostles. You cannot get much more direct in opposing God. Saul had power, and used it; but this man was completely turned around by God and became one of the most famous authors of the Bible (Paul) and traveled across much of the known world spreading the word and love of God. He's a huge example that God can use the most severely broken for great purposes... if we let him.
Letting God do things in your life can be difficult to do and to understand, especially for those who don't believe the Biblical God exists. I once heard a metaphor for Salvation and Humanity that has stuck with me for a while: A man is sitting at home in his chair. He has a fatal disease that will kill him, and he knows this. A cure for this disease is discovered, and a friend of the man's comes to him and says "There is a cure for your disease! Come and get it!" The man says "no" and continues to sit in his chair.
There are many different reasons why we say no to the cure. Perhaps we don't believe the cure actually exists, or perhaps we will not admit we have the disease. Maybe we are scared of what it would mean in our lives if we actually accepted the cure... it sure would change much of what we are, and what it means to be human. At that point, to be honest with ourselves, we must ask "Am I truly happy and contented, where I am now? Do I really need this cure? At this point in my life, do I have an unshakable feeling of security and fulfillment inside?" I can personally say that I have that, only because of God. God has placed it there, and I want to do my best to have that be seen by others. He is building it out of my past and future failings and successes because He wants to use all of me. That's how much of me He loves. That's how much of you He loves.