My anxiety, fear and despair spiked today while Mono was at work and I reminded myself that I will not fall over, I will not vomit, I will not lose everyone and everything...
RP, this is very extreme. An actual feeling of nausea and of losing EVERYTHING because Mono has chosen to be poly, it just doesn't make rational sense, although I know it comes from deep within your psyche.
I recall going through a serious depression when my ex h got a gf... Even though we made the decision to try polyamory together, the reality of it turned out to be so unlike my imagining (so unpleasant), I totally lost it.
My husband saw how low I had sunk and signed us up for marriage counseling. Our counselor almost immediately diagnosed me as having clinical (situational) depression, and recommended an antidepressant for me, which, along with therapy, helped a lot. It sounds to me like with your extreme feelings of loss and being abandoned by everyone in your life because of the actions of one person... maybe you need some professional help? Individual therapy? Couples counseling for you and Mono?