When it rains it pours. I'm sorry that you had a rough time. Breakups do tend to cause some fallout and that's not uncommon.
A good way to insulate yourself is to date outside the polycule whenever possible and avoid dating people's support structures (friends/family). Set some realistic boundaries for yourself. For example, I know sharing multiple partners with the same person is hard for me so I choose not to do that.
One thing I appreciate about polyamory is that you learn to love on new ways from different people. It can really improve the way you show appreciation for existing partners. The different perspectives, tastes in music, hobbies etc open up a whole world to me that would be otherwise unavailable.
There is so much more but I don't want to write a wall of text lol