Hello and welcome. I am so sorry for what brings you here, but this is a place of great support.
Good for you for kicking your own addiction. Keep that in front of you at all times, it has to be first. Secondly, I would not keep this to yourself. You need face to face support from those who love you, and keeping something like this a secret is mentally damaging for you. You don't need anymore of that. Addiction breeds secrets, and that's toxic.
Your daughter is obviously on a slippery slope. Of course she feels nothing because she is taking Xanax. It keeps you from feeling anything, which is what she wants right now. It's also highly addictive, and has a fairly high street value, which is why she is selling it as well. It's a benzo, and mixed with alcohol is especially dangerous.
I don't really think there is a point in letting her deny it. You know what you know. If she is not receptive to help, she won't get it. You cannot force her. What you can do is sit her down, tell her you know, don't put her in a place to have to lie or deny, because she will. Tell her you love her, tell her you support her, tell her you will help her in any way you can. And follow through. If she will let you, great. If not, then you will have to decide what your next steps are.
While my qualifier are not my children, I can say that getting therapy from a counselor who specializes in helping families with addiction would likely be a great start for YOU. Also, Naranon would be good so you have face to face support.
Keep posting, you are not alone.