I'm really feel for you. I'm new to poly and I'm not sure how qualified I am to speak on this, but I'll do my best to give you my perspective.
a. You are not psycho to want to disclose what actually happened with X. I think she should know, but this should be done without judgment on her, as she is not the one who broke any rules.
b. Relationships do ebb and flow, and I also believe it is possible to fall in love with someone multiple times, (necessary even) but this would be a deal breaker for me. Or at least it would take some serious action to repair the damage done to the core value of the relationship... trust.
c. I believe one of the most honest and loving gifts one can give is forgiveness. Everyone is flawed and deserves to be seen as the imperfect human they are, but there is a fine line between forgiveness and being a doormat.
All the best to you as you wade through this. You sound like a lovely person and you deserve an honest love.
Concern yourself less with love and more with loving.