Sympathies. The more I delve into poly stuff the more I realise that everyone's situation is totally unique to them. One thing most people seem agreed on is that communication, open-ness is paramount. Not necessarily that everything has to be told, but that you need to agree between you what can and needs to be divulged. So you've started off on kind of the wrong foot, but I have seen it repaired. It will take a great deal of work on your part to work through your anger/betrayal though. Trust is pretty difficult if its been broken. Good luck (I know this wasnt advice, but the amount of threads in here are too many to count about this stuff so it was just a sympathy note)...
Louisa (website addict, entrepreneur, founder of The Vagina Times
& Multiple Match
among others...genius at implementing ideas which are apparently not profitable)