I'm sorry things ended badly for you, Rayek. My best advice, learned after a number of break ups, is to feel what you are feeling and not try to control those feelings. Someone here compared grief to a roller coaster. (Sorry I can't remember who.) You don't have much control over where it will take you and trying to control it only prolongs the pain. I know you have to hold it together at place like work, but try to find a safe place where you can mourn - alone or with someone who cares about you.
I understand why you don't want to give polyamory another try. But I'd encourage you to look at it again when you have finished grieving. It might be for you and it might not. If you do try it again and you are in a primary relationship, you might consider setting a boundary that you must be allowed to meet your partner's new sweetie. My primary partner and I have found this to be a helpful thing for us. He is my touchstone and can often see things about others that I don't, especially if I'm in an NRE or lust-filled fog. He's never vetoed anyone, but he has brought certain things to my attention that were helpful.
I wish you well.