I know how you feel, i was left behind for a different relationship in my first poly relationship. I am still trying to figure out if I am cut out for this myself. I think first thing is be gentle with yourself. You are just learning about this and you can try again when it feels right. I haven't decided if I am ready to move on my life, I half-assed tried again but it hasn't really been what I wanted. I do think I am very lucky in my primary at least though.
Just remember what you DO have and work from there.
I hope you find the forum helpful and want to wish you a warm welcome here.