Poly is always a struggle at first. We live in a (supposedly) monogamous society, so we are all pioneers, forging new paths. Few people really understand what polyamory is, or how it should work. That's why research is so important. Many people don't do research and just play it by ear, and this can cause disaster, depression, anger, jealousy, envy, etc.
Despite society being "monogamous," it's clear humans aren't meant to be monogamous. There is way too much going on to make up for being mono. Even romantic movies and books will give us an outlet for our desires for others. Then there is porn, cheating, and serial monogamy, and divorce.
Read the book Sex at Dawn to see how society used to function before civilization and the patriarchy caused us to have to live monogamously (or at least try to, or give it lip service).
All that said, poly does need certain guidelines to be successful. Most experienced polyamorists know that their limited time with each partner should focus on quality time. Calendars are important. Sex labs should be done regularly, and condoms are a must, unless strict parameters are met. TMI sharing is generally a bad idea. Each partner deserves its own privacy.
Go to the website morethantwo.com. They have lots of tips on common mistakes and all the issues that come up.
I am in a hetero MFM V, the other man in the composite relationship is just involved with me as a platonic friend, and we are both romantically involved with the woman. All three of us know this and are consenting. We also have an agreement that all three of us are primary partners. There are no secondaries in our V. We do also have an agreement that none of us will venture outside our V to have sex, this is just an agreement for us and would probably not work for most people.
Poly did not come easy for us at first, it was a struggle for all three of us, even though we strongly believed in poly (and still do) as a rare and wonderful ideal. But we did not know how to make each other happy in the beginning. There was jealousy, and, as we later realized, a shortage of privacy. We are kitchen table poly, but now with notes of parallel poly. I think poly is hard for most people in the beginning, it takes a while to figure out what works best for each of us.