rp, very sorry to hear that things are a little rocky right now with you and mono - I so enjoyed meeting you two all those years - you are a great couple.
If I may offer a different perspective in your quest of screaming "WHHHHYYYYYY?" to the ether.... (and by the way, if I were in your situation, I would more than likely be doing the same).
Mono has always stated that he is monogamous. He has been very very open about it. Monogamy means that when you fall in love with one person you automatically must fall out of love with the other (right? cos that's how monogamy works). You knew this, and have visualised your relationship with him on that basis.
So I think that it is only natural that, when he finds someone else he cares about, you see this as a threat to your relationship with him. Has to be... he's monogamous, right? Regardless of what YOU feel or how poly YOU are, you know that HE isn't. You are interpreting his actions based on that monogamous mind-set.
Mono is an intelligent and very self-aware guy - you and he have shared so much with each other, and I'm sure he can understand why what you see as going on is upsetting to you, no?
Often the gut feelings of folks being distant are your subconscious trying to tell you something - in this case, that he was hiding something. Trust your gut on stuff like this, for sure.
I hope that the two of you can continue to work in the stellar way that you normally do.
Virtual hugs from across the continent.
"Listen, or your tongue will make you deaf." - Native American Proverb