Welcome! I am sorry it turned out how it did.
I see where you could have been more honest, but at the same time... Her saying it is too late for you to express yourself... Sounds kinda silly to me. People are always evolving. I think people have a responsibility to speak up and "report the weather" so others can know how they are doing.
It is not like anyone can mind reader others to know!
If after a month in she said she could not trust you, and it is like you two gang up on her... Why did she stay another 6 months? That makes no sense to me.
Were you actually ganging up on her on something? Or was it that you each had a reasonable opinion about solving something and she did not like reasonableness and taking time if it means she does not get her way NOW?
I am not sure what "telling him everything" actually means. It is fair to keep some things private between you and her. Or him and her. Or you and him. Each mini relationship inside the bigger polyship has to be healthy. You guys figure out where the privacy lines are. It is not healthy to be enmeshed with no personal boundaries.
It sounds like she wanted "instant intimacy" with you. And mad you expected to "build intimacy" instead?
Or she was a cowgirl trying to rope one of you off for just her by dividing and trying to break things up? If she was cowgirl then her staying another 6 mos trying makes sense to me.
A triad is a hard model. Going from zero to essentially three V's stacked together is a lot of layers.
I would suggest reading. You could start here:
http://www.practicalpolyamory.com/downloadabledocuments.html
http://www.kathylabriola.com/articles
http://openingup.net/resources/free-downloads-from-opening-up/
https://www.morethantwo.com/polyamory-basics.html
It is by no means exhaustive, but those "hub" pages lead to a lot of articles.
Galagirl