Originally Posted by mostlyclueless
It was a disaster. My partner and the girl he was seeing were both very understanding and patient with me and both complied with any request I made immediately (e.g., I want to meet her, I want to develop a relationship with her on my own, I want you to take a break). But I think I can say it was one of the lowest points of my life. I was in constant pain, completely irrational, eaten alive with jealousy and sadness, and all for no f-ing reason. Nothing was taken away from our relationship or damaged by it, yet I was completely devastated. I just felt shattered.
There was a reason, maybe not a good, rational, logical reason, but there is a reason(s) for your pain and jealousy. Did you ever try to understand why you were feeling these feelings? Did you fear his loss? Did you feel 'less'? These are some of the more common feelings behind why people feel jealous. In other words, what was the emotions driving your jealousy? If you truly don't know, then jealousy can't be easily addressed, even when everyone involved does everything right, like what you described. It will be difficult, if not impossible, for you and your partner to proceed with poly until the underlying causes are brought into the light.
And, yes, ultimately, it could be that monogamy is a better choice for you than polyamory. You could be monogamous with a poly partner but you would still have to deal with understanding and managing jealousy.