Welcome. I'm sorry you're struggling with your son.
I agree with Somewhereoutthere. In addition, I've found it helpful to NOT ask myself how or why. There are no answers to those questions. You'll just cycle thru guilt, looking for someone/something to blame.... which is often ourselves. It's a typical response we've all gone thru, however, it isn't helpful. Mostly what it does is torment us.
It does feel like a sentence, that's for sure....but we really do have more power to heal, recover and mend our broken hearts over time......
Watching our kids go off the rails when there is nothing we can do to prevent it is devastating. It is unbearably sad. It was the most difficult thing I've ever been confronted with, I know how you feel. Remember, you didn't cause this, you can't fix it and you can't control it. You're powerless.....which feels horrible.
Do kind and nurturing things for yourself......take really good care of yourself now.....we're here with you, you're not alone.