I am not sure how useful my response will be -
There are happy endings. Sometimes there are unhappy endings. My ex-husband and I became poly, divorced due to things totally non poly related but that being poly kind of brought to light. Married a second time to the guy I had been dating for 6 months at the time we separated. Mono for awhile, poly again for awhile - of course with me, I have actively wanted to be poly in all these situations, so for me things are going well. If you want to actively not have poly in your life, or struggle with it a lot, maybe you won't have a great outcome. Keys for me are communication, honesty, love and actively trying to be understanding and work through problems.
I am sorry women have told you that you're not good enough for them. Screw em? If she isn't telling you this, then enjoy. I'll suggest reading books on self esteem (good for everybody) I don't worry that I'm not enough for my husband or my boyfriend (I mean, the former has a gf, the latter has a wife and a gf, and I have a husband a bf and somebody else I'm dating - that all contributes to a lot of happiness for everybody, but if I was mono again tomorrow, I'd still love my husband just as much.) I love my husband, my ex, my best friend, my bf, and my sister, and they all contribute something wonderful and unique to my life. That's why I don't worry that when I am with them they wish they were somewhere better than with me. I am guessing your girlfriend doesn't think anything dissimilar. She is with you because you make her happy as hell I imagine.
One non poly focused book which I always find helpful is "If the Buddha Dated: A Handbook for Finding Love on a Spiritual Path" by Charlotte Kasl. Whenever I get stressed or panicked or worry I'm not enough, I open that up and find something to calm me. I can't recommend that highly enough to everybody.
So poly meetup - sure go! If you are feeling stressed out, sit back and be quiet. If you can handle it, think about if you might be able to be interested in anybody else there, sit with if you might be OK being interested in a second partner, figure out if the idea repels or attracts you. It will give you food for thought on if you want to be mono or poly, let you see interactions between other poly so you have a better idea what makes you comfortable or uncomfortable. No hurry to act or decide anything. I attended a poly potluck, and kept quiet and listened mostly, and it is OK. Just do what feels natural to you, if you feel extroverted great, if you don't, don't sweat it. If the idea of being there stresses you a lot, you can always drive separately and meet out front, so if you need to escape you aren't stuck staying there longer than you're comfortable.
Happiness will never come to those who fail to appreciate what they already have.