Look, you both have a lot to digest here. That is understandable. If you guys could do it and solve it on your own, there wouldn't be this struggle. That is understandable.
If you have come to find you have nobody to turn to and you have cultivated no other close friendships to share your joys and burdens with in Life -- perhaps that's telling you something here. Your wife is not your best friend from among many friends. Your wife is your best friend from being the ONLY friend.
That is a hard burden on both -- you have no other support system when the fit hits the shan. She has to be your everything even when she herself is not at her best.
What if the issue were not poly? What if she were in the hospital with a stroke? Where would you go for support in that hard time? Why can't you go there in THIS hard time? Sometimes the spouse is NOT the guy. Or rather, not the ONLY guy.
How about accepting that you are each too close to the issue for comfort, you both are under burden right now, and both are better off seeking a third party person like a counselor? Minister? Who do you guys normally see for big stuff processing? Either to talk to independently of each other, as a couple, or BOTH? Loads shared are lighter.
She's got to process her own stuff, not process hers AND guide you through yours. Her reaching a limit that day on that verbal spew? That doesn't mean she's abandoning you. But I could see her saying "suck it up" like "sheesh! I'm dealing with hard things here too! Everyone put their own oxygen mask on first! Stop acting out at me! I hurt too! Suck up your own part of coping! Everyone cope!"
You both could be emotionally flooded.
If that happens, don't metaphorically pee INWARD on each other in the relationship and escalate tension. Pee OUTWARD to de-escalate tensions and lower the volume/stress. To others who can help and support in appropriate ways.
How are your conflict resolution
skills with wife? Strong? Weak?