My son also grew up in a loving home. I've wondered why he would need to medicate himself with drugs and alcohol, too. He didn't see it modeled by his father and me. There is something going on with him. He has indicated to me that he has social anxiety. His counselor believes that there is something in his past, some kind of trauma. Perhaps social anxiety is all the trauma a person needs to propel them into unhealthy waters. I don't know. We may never ever know.
My son's drug and alcohol abuse as a teenager is what brought me to rely on God in the first place 14 years ago. My son abused drugs and alcohol all through junior high and high school and it was a very trying time for me. I had my own depression to deal with then. My husband and I moved out of state to a new city and my son left his abusing drugs and alcohol behind for five years. I thought it was over. I thought I'd never have to revisit those days again. I thought God had delivered my son from a bad situation when we moved and he seemed to be doing so well. Surly since he was well those five years he was always going to be well. Then my son decided to move back to where we used to live. I remember sitting down with him and praying with him that God would protect him--that we had moved in order to remove him from bad influences and for him not to return to those bad influences. I don't think he thought he'd get back into what he had left behind, but he did. He started right back where he left off and became a heroin addict.
It's hard enough to go through life but without trusting on God would be worse. A lot of people think God only gives us as much as we can handle, but I don't believe that is true. If so we would never turn to Him. We as parents of addicts (and others who love someone who is an addict) have been given much more than we can handle alone, but with God we are not alone.
My son is back with his father and me and we are cautiously hoping for the best. As difficult as it is to revisit old wounds, I choose to believe that God will use evil for good.
Hang in there.