lovehoops, would writing a letter maybe help, and have her read it before she starts on a tangent? Maybe just try to calmly and lovingly explain to her that berating you and demanding answers isn't going to help anything, and if anything, will just make things worse.
Tell her she doesn't need to fully understand. She just needs to know you acknowledge you have a problem, it's extremely difficult to quit, but you are taking all necessary steps you can to quit for good. Then give her the letter before the talk begins, with hopes she might go into it a little calmer and more open minded?
All the best though!