Dropping the words, "you have learned not to trust her", might be for the best.
When stating a boundary, you should not use the words, "I will not allow you". B/c those are fighting words to a wayward. You can't control what she does. A boundary is about your actions. "If I am being yelled at, then I will hang up the phone". (Not the best grammar, but you get the message). See, she is free to do whatever she wants. You are just telling her what YOU will do if she decides to start yelling.
It is not about what you feel should work in your M. It is about doing the work that gets the right results. Do what works!
