My first poly breakup was really harsh. I broke up with my husband a yeah and a half ago, and following it my boyfriend also broke up with me. (first because breaking up with my husband was making it harder for my boyfriend and me to be together, second because he felt he couldn't trust me now that I had broken up with one of my partners, and that I might do the same to him.)
Ultimately things between my boyfriend and me got fixed, and he supported me through the rest of the breakup, and to some extent the breakup is still going on since we no closer to being divorced despite all the lawyers I've talked to so far.
What I can tell you is that being rejected is a bit easier when you have other partners. You can't think it's you who are not attractive enough or unlovable something, since you already have one or more partner(s). That definitely helps.
My first poly breakup was also my first breakup (I never had any serious relationships before) so I can't compare, but it was devastating for sure. I'm glad I had the support of my boyfriend (past the first reaction, he was extremely supportive) and that of a few friends. I still miss the ones I lost in the process though