Tara welcome. It may be prudent for you to take down the picture of your family, since this is a public site and we need to protect our children and ourselves from identification.
Many of our kids have a 'conduct disorder' or a mental illness or as in your son's case, are substance abusers. You might also post over in the Substance Abuse forum where other parents are dealing with kids who use.
It isn't about you, or about you screwing up, or someone being lenient or strict, although some of that might come in to play, it is usually our kids who make poor choices and refuse to take responsibility for those choices.......At 19 your son is considered an adult, and an adult is supposed to be able to act in a fashion which shows self respect, respect for others, the ability to make positive choices, to be responsible for themselves, work or go to school and eventually become a contributing adult. However, for whatever reasons, our kids don't.
At the bottom of my post here is an article on detachment which you might find informative. There may be nothing you CAN do, since your son is legal adult. That is the hardest thing for us parents to understand, that we cannot make any difference in the choices our kids make. We are essentially powerless. We have no control over what they do. It is a bitter pill to swallow.
Most of us here have gotten ourselves in to a supportive environment to learn how to cope with and understand what has happened to our kids. You might try a 12 step group like Narc anon if that feels appropriate. Or a therapist or Families Anonymous, or a parent group. Someplace you can go to get support for YOU and your husband.
You cannot reason with a substance abuser, it is an act of futility. It will make you crazy. This happens to smart and talented kids, poor kids, rich kids, privileged kids, street kids, pampered kids, kids of all nationalities and backgrounds, there is no answer to the why of that.
This is very hard on us parents. When our kids go off the rails, we feel guilt, anger, sorrow, disappointment, grief.........many, many emotions.......which we have to come to grips with over time. It isn't easy. Which is why I always advocate finding some kind of a supportive environment for yourself. You will likely need it.
It's helpful to continue posting, we have been in your shoes......some of us still are. Hang in there. We're here if you need us. You aren't alone.