Welcome to SR. Many of us are speaking from our own experience in this.
Were you thinking you could do something? Were you thinking that because you had that conversation with her about drug testing her, that was why she decided to move out? If you do feel like you were too rough on her, are you thinking you should rescind and tell her she can stay, no matter what she does or does not do?
My advice is the next time you see her, cup your hands gently on her face, look in her eyes with compassion, and tell her that you will always love her but that until she is ready to leave her drugs and alcohol behind, you cannot allow her in your home.
But dear Softball, you may not be ready to have that kind of conversation with your daughter. So in the meantime, read everything you can about drugs, alcohol, the effect on the brain. Educate yourself. Go to Alanon and/or Naranon meetings. And somewhere along the way you will be able to formulate the answer you asked in your post, "What should I do now?"
Whatever you do, keep coming back.