I'm sorry but I do not think "caught up in the moment" is an excuse. I've been on the recieving end of that and it was bullshit. I could get caught up in a moment but I do not but myself in a position that has the potential to lead to crossing boundaries that have been set. I try to put myself in the other's person's postion more than most do. I think that is because of all the wrongs that were done to me in the beginning of this relationship. I will tell you that one of the absolutely biggest hurts I've been dealt was form the three of them together and they used this excuse. Yet two of the three will tell you that I entered their heads more than once while doing what they did. Tells me they knew they shouldn't be doing what they were.
Enough of that. Other than to say...the two that thought of me and will admit they knew at the time I would be hurt, have since asked for forgiveness. The third, well, she has tried to make my pain somehow reflect that I was treating her wrongly. Kitten never likes to be held accountable and will always try to make something about her.
Then there's the fact that she lied to you. Ok, she did this. But when you asked her, she denied it. A lie is one of the worst things you can do to me. And lies of omission can be just as bad. Some people just do not like to admit they are lies as well.
No remorse...to me that means she would be willing to cross boundaries again for her own pleasure regardless of how it will make you feel.
I'm sorry to be so harsh. Counseling is probably your best option here. Thankfully, you are going to seek that out. I wish you luck. I will tell you that the issues you are facing are not due to being poly. They are issues faced in monogamous relationships as well. They could be described, I guess, as personality issues.
Again, good luck and let us know how things are going.