It's very brave of you to step back and take action for your daughter's future. You are a good mom to do that. She will not continue to thrive with active addiction or deception in her daily home life. If it were me, I would explain it to her by using a disease or illness metaphor. Reassure her in any way that makes sense, no sugar-coating but no excessive details. As time passes, she will ask more and you can answer her with more depth as she matures. I am sorry for your pain, very sorry, but try to comfort yourself with the fact that you are doing what is best for your daughter and YOURSELF! Take care.