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.