Here's my suggestion.
First, go to your boss. Explain that you've had issues with weed in the past, but you have stopped and want to do what you can to keep your job. Tell your boss that you are clean and intend to stay that way for good (you are and you do, right?). Offer to take regular drug tests if necessary.
Second, go to your husband. Beg for forgiveness, whatever it takes. You are sorry. You messed up royally. But you are also scared and you are being bullied. If the marriage ends then, yeah, it'll end, and that's unfortunate, particularly for your child and the one you are trying to adopt. But you will have told, and you will have given your husband the choice to stay with you or not.
Then go to the cops, and have them deal with this sack of **** who's trying to blackmail you. If he rats on you to CPS, then you have backing from your boss that you are turning your life around.
I am suggesting your boss before your husband (and, really, one should be on the heels of the other) because retaining your job is going to come in handy if your husband leaves you or CPS is called or you need to fight for custody.
I am also suggesting telling, and doing this, because this is the kind of thing that cuts blackmailers off at the knees. This asshole is banking on your silence. Speak out and remove his power for good.
But you need to line up some ducks, to protect both yourself and your family, first.