Broken trust is something that everybody needs to acknowledge and strive to mend. It can be mended, though, especially if there is no pattern of betraying trust and if everyone involved wants to work at it.
Your inability to express your misgivings is very very natural - I think our society tends to have an attitude of encouraging folks to "keep it tin" and to "suck up and deal" - those are habits that need to be broken in any relationship, whether monogamous or poly, as far as I am concerned. There are many ways to do this, depending on the nature of the three of you (amongst others whether you each tend towards extroversion or introversion) - one way is to actually schedule a group meeting at regular intervals, and encourage everyone to talk about how it's going (not just the bad stuff, but everything) - people should never be punished or told off for having negative feelings. I feel it's important to generate a team atmosphere - the feeling that you are working on stuff together.
What your husband did is very definitely breaking the agreement that was in place - and I'm sorry "I was drunk" is no excuse. You need to re-examine your "rules" that you have in place - either you can keep them without problem or you can't. If you can't then it needs to be renegotiated.
So, when you say that your rule was to "play together" does that mean that there is no sex without all three of you being there and involved? So you and your husband aren't having sex without your girlfriend present?
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