Welcome to the board.
A couple of thoughts,...
I don`t think you can go through this kind of growth as a person, without feeling pain along the way. Many of us get ourselves in trouble, and confuse communications with our loved ones, when we worry to much about hurting them.
Her having ' No idea about our plans to take this poly'
With this statement, I would expect her to be angry at first. She could very well be upset, that she, as the gf, is now on the outside looking in. Since you two have this plan, and she is the last to know.
Own it, and let her feel her emotions over this. Explain how it came to be that she is the last to know. Be honest.
Make sure to have some type of dialogue, where she feels part of how things will go, and how communications should be dealt with in future.
Let her know, it is very important to you that everyone move forward together.
Best of all, good luck with your future relationship