Monkeystyle shares a lot of wisdom here, RP.
As some others of us have said to you before, I think your biggest issue regarding all these developments with Mono's process in branching out to poly is the loss of control you thought you had. You were at the center, the desired one with all these relationships, basically the woman at the helm. You were even Mono's domme for a time (don't know if that has changed - you haven't written about that for a long while). But now everyone in your sphere is changing, wants more, has developed attractions for other people (even Derby and PolyNerdist!), and what has become evident is that you're simply not in charge anymore. Even the man who claimed he could never be anything but monogamous (which you always said you could not understand) is embracing poly, and essentially, as I have written before, your long-held dream of a poly family/tribe is actually coming true. Frankly, I would think you would be excited and definitely more supportive of him wanting to be poly.
Of course, Mono could have been more vocal about what was going on with him, but to call him a cheater seems rather harsh when he was simply developing a friendship with a woman while you were developing a romance with another man. He tried to control his feelings for that woman but could not. How many times have we all said, feelings develop on their own. But you are making Mono out to be some terrible villain who has betrayed you, though he has stayed by your side and tried to get you to see that he is his own man just as much as you are your own woman.
Yes, it is perfectly understandable that you are grieving the way things were. But I think your grief is very closely tied in to your self-esteem, and that it is your sense of self-esteem that is what you need to work on. Because it seems (from the outside, anyway) that yours was very connected to being the one that everyone focuses their love and attention on. And craving that kind of attention speaks to an underlying belief that one does not really deserve it. Now you are sharing that spot, and your loves have other people they love and are dividing their focus - it's not all about you anymore, but it isn't not about you either! So, how can you still be confident in your self-worth as you share your position with others, and not let it shake you so much that the people who were so devoted to you are also devoting themselves to others? That is what I see will be crucial for you to confront and resolve within yourself. But you might lose everything if you continue on the track you've been.
Cleo wrote something in her blog regarding some changes with one of her relationships (he is now seeing an additional partner, too), and I think it is very relevant for you:
Originally Posted by Cleo
It is my responsibility to find the new balance FOR ME. The fact that C is dealing with it differently does not really matter, I have to take care of me and of how I deal with it. The change is happening no matter what and there is nothing I can do about it. It is my false sense of power that is causing the most anxiety. But yeah, accepting the change is hard.. especially if you are not the one who made the choice to change things.
There is such a thing as over-processing what we are going through, to the point where we stay stuck and start blaming others, and don't make any progress. At some point, what it takes is just a deliberate "no" to self-pity, a letting go of the need to be in control, and a concerted effort to make a shift in one's perspective.
I'm rooting for you!