Have you put M in touch with this board or any other way to read about polyamory and particularly people who practice it? I'm guessing she has so much guilt and shame around her feelings and actions, being honest is more than she can handle. It's one thing for you to give her this amazing story that you understand her nature and will try to accept it, and another to truly believe that you won't despise her for it, especially if she despises herself for it. It could be so reaffirming for her to hear stories of other people who love the way she does. I know it was for me.
You might also need to clarify between you what honesty means. Maybe you do want to know who she's sexually involved with, but not the details. Maybe you want to know whether she has other partners and how many, but not who they are. Considering your jealousy when you were out together, I'm guessing you'd be someone who'd like to know at least who she is involved with or interested in, so you aren't in a panic over every person who looks her way.
Hang in there! She's not deceiving you out of disrespect or malice. She's probably just afraid of losing you, most especially now that she realizes you might be the first man to ever try to accept her the way she is.
Married to a monogamous man 15 yrs, mother of 2, dating C 3 yrs, and in a romantic friendship with L more than 20 yrs