He seems to feel he needs some kind of mind-altering substance in order to feel good about his life. If it's not booze, it's pot. If it weren't pot, it might be something else.
That's his life, and his choice, not yours.
Your only choice --- as hard as it is --- is to decide whether you are happy living with someone who is so unhappy that he constantly has to change his brain chemistry in order to feel okay about life. If you do not respect or trust him, that's a huge warning sign that perhaps you are "settling" for this relationship because you are afraid of the alternative.
And of course there's the kids. Raising kids with a pothead is not something I would choose, but it's darned difficult to put your foot down and say, "We are NOT doing this any more." It took me a long time, because my fear was still more powerful than my revulsion. He was a habit. When I finally did separate myself, I found that I was so much happier being around people who didn't need drugs to feel good. We were together 7 years, not 20, but the habit we develop is the same......we have the break the habit in order to make the right choices.
Therapy helped me to separate myself and my fears from "his" behavior ---- I hope it will help you find a similar serenity. Welcome to SR. It's a great place.