You've been given lots of help, I'm mono so maybe I can give some additional perspective.
Firstly I know what it's like to be fine about open relationships until you find yourself in one with someone that you love to bits. I was fine with my husband having other lovers, but with my new partner it was a completely different story. My ability to handle it was obviously very dependent on the relationship and you just don't know until it happens. I also associated the desire for other relationships with the primary one being not the best. So it was a real shock to discover that even in our beautiful, sparkly new relationship, Z still loved his SO. I was even fine about it philosophically but emotions don't seem to take that much notice of philosophy.
Secondly I know what it's like to want to pull away and stop the pain. I threatened this a lot but as someone said, love is a really hard thing to pull away from, and I never managed it for long. I analyzed this behavior and what was actually behind it was that I wanted Z to really feel the loss of me and happily give up his polyamorous ways.
Dealing with this is a process, the length and success of which is different for everyone. From your reaction it sounds like you're purging some pretty heavy emotional stuff. Not all of this will be about what your partner, it's just been triggered by your partner. It certainly wont all be put right in a few days or even weeks. It might be really hard to see it now but looking back on my process I can honestly say it has been one of the most significant opportunities for personal growth I've ever had.
Nurture yourself and accept all the love that is on offer.