Welcome to the board. As others have said, there are dozens, if not hundreds here (including lurkers) who are going thru what you are.
Other posters have given you some good information and support. I hesitate to say advice, because no 2 people are alike and no 2 will process this in the exact same way.
I will address one thing you said. That you didnt like dating (understandably, there are a lot of losers out there), found your wife, she was The One and you felt "settled."
But marriages don't just get settled and stay that way. You've only been together less than a decade? Try a 30 year marriage (like I had and am now out of). People have experiences, thoughts, joys and pains, they change and grow. Sometimes it is thrust upon them. Like, having a child, or several kids. Like, needing to take care of an ailing child or older relative. Sometimes one partner becomes ill. Or you might need to deal with you or your partner going back to school, becoming obsessed with a new time and money consuming hobby, losing a job and self esteem, etc etc.
The thing is to not become insistent that neither partner is allowed to change, or to uncover parts of themselves that they buried to seem "normal." One has to roll with the punches and learn to look at each change (your own or your partner's) as an opportunity for growth and further enlightenment.
It's good you're on meds and in therapy, taking responsibility for your own life changes and feelings. She kept her poly ideas to herself until you were more stable. That speaks of her love and caring for you. Now, it's your turn to be kind to her and look into how you can support her in being her most authentic self.
You might find comfort in the fact that many here who were blindsided by their partners coming out as poly do not lose their partners to the new love interest. Quite often the marriage is indeed actually strengthened by the talking and openness required by exploring polyamory.
Love withers under constraint; its very essence is liberty. It is compatible neither with envy, jealousy or fear. It is there most pure, perfect and unlimited when its votaries live in confidence, equality and unreserve. -- Shelley
Mags (poly, F, 61) loving miss pixi (poly, F, 39) since January 2009, living together since 2013
In a newish relationship with Steve, (34, poly)
"Master," (mono, 34), miss pixi's Dom for 3 years