Virtually every person who walks the poly path runs into (problems/challenges/opportunities). Until fairly recently, we pretty much had to figure out solutions on our own...or have our partner(s) impose them on us, which may or may not be beneficial.
If you've ever wondered what kind of advice polyamorous people give and get, well, here you go. There was a thread in r/polyamory asking "what's the best piece of advice you've ever received?" and the replies are enlightening.
Emotions come from a very old part of your brain that's trying to motivate you towards a certain behavior. Jealousy = "Danger! Danger! Protect your stuff!" NRE ("new relationship energy") = "Hang around this person all the time and make babies!" Etc.
The rational, decision-making part of your brain is totally separate. So you can say "Hey, thanks for the warning emotions," and then choose what you want to do with that information. Your emotions aren't what you HAVE to do; they're just a suggestion. They're not bad or "less than" the rational part, but they have a job to do that's not always conducive to your true happiness. I like to think of them as an overly-sensitive smoke alarm!
One of the reasons I am well-equipped to handle polyamory, relatively speaking, is that I have always been self-reflective. When I feel an emotion, I can almost always tell you why (though sometimes I may not want to admit it). If I can't explain my emotions, I get very nervous, very quickly: I feel as if my mental ground has shifted from underneath me.
This allows me to process my emotions reasonably well, most of the time. Of course, there's the trademark Breadner knee-jerk reaction to change, which isn't pleasant to experience or behold, and in my case it's compounded by an overactive "what's the worst that can happen? gland. Thank Trintellix that last doesn't squawk quite as loudly as it once did.
At this point, a year in, it's hard to rattle me. Which I think is a good thing.
Relationships aren't owned, they are experienced. You don't own your friendships, your romances, your long term partnerships. They aren't possessions, and they aren't reduced or destroyed by other relationships. You don't own the connection between yourself and your partner, you create it every moment you are together.
Put this one in the column of advice I've never needed. To me, this is the emotional equivalent of two plus two. If you believe you own your partner, that makes your partner a slave.
She/He didn't do that TO upset you.
She/He did that AND upset you.
Now this, on the other hand, is advice I continue to require almost daily...and I have a woeful feeling that other people consider this to be kindergarten-level stuff.
I've actually been rejected in a poly context, and the pain from that lasted roughly forever. Maybe two forevers, I kind of lost track of time there for a bit. I let it render me an emotional wreck. Had I the maturity to actually take the above to heart, I would have been much, much more resilient. We really do choose our emotions--or, given the first advice-nugget, our response to them--the same way we choose our loves. and I'm going to resolve never to forget that going forward.
"You were already in two relationships. Your relationship with her, and your relationship with yourself. You already know how to balance those two, how to keep one from being a negative on the other. She's the same way. She just knows how to balance three or four. You'll learn. You may never balance more than two... but you'll learn why she does."
"Mono-Guy" is something of an r/polyamory celebrity. As you can infer from the above, he is the monogamous part of a "mono/poly" relationship. Such beasts do exist, though you'll trek far and wide to find one that works as well as his does.
I have a friend like him, a person who truly gets polyamory despite being monogamous. It's really humbling to always get solid and sound advice from such a person. You find yourself thinking geez, if monogamous people can saw right through this so easily than I certainly should be able to. Someday I'll actually meet another polyamorous person, for offline values of "meet", and I can onlt hope she'll have that mental toolkit.
Change is inevitable. You can hold on to feelings and let them fester or you can release those hang ups. Be okay with things not being what you thought they should be.
That's good advice for anyone, mono or poly, and it holds true for all of life, not just romantic relationships. It's also something that I once had a great deal of trouble with, something that, again, Trintellix has helped me banish. "Festering", great word. Derives from the Latin for "ulcer", and sure enough, if you let feelings fester that's something you might end up with.
And finally, because I can't resist quoting this song...
you love to hear me sing, even if you didn't write the note
i love to hear you laugh, even if i didn't tell the joke
you know i love to cuddle, love to pull your body close
and i love it when you're happy even if i have to let you go
cuz if you need your space then baby you can let me know
i'll you from afar you my star in a telescope
i'm not a god, i'm not a fool but I would be both
to think that love was something i could control
--"Can't Help But Fly (The Poly Song), 'naimainfinity'
Truer words were never sung.