You seem like such a nice person! I can tell that you are really trying to do this the right way and you care a lot. Admitting that you're having jealousy can be difficult, and I admire that you are trying to work through your situation. Second, I agree that perhaps rather than straight up jealousy you're realizing that you and she are in a different place in regards to your needs and your relationship goals.
Honestly, I've been poly since I was a teenager (now almost 30) and being with a partner that was going out and sleeping with that many people would bother me too. I would question his / her motivations for relationship, and question what they really needed. I would also wonder about their self-image and how much they respected themselves. Why would they be doing this?
I'll tell you why... I was that gal when I was a young teenager. As soon as I hit puberty I had sex with just about everything that would let me. Before I was 20 years old I had sex with over 100 people. Was it fun, or beautiful or special? Not really. It felt great physically and I would get on an emotional high from it... but it didn't last.
Once I 'grew up' and started respecting myself and my partners I found that I wanted to be able to put a lot of energy into my relationships, and that sex between me and someone I loved (I met my husband when I was 20, so this refers to him) was completely sacred, beautiful, and beyond anything I had ever imagined. I definitely changed my tune.
To me, being polyamorous is more about the connections with people and the love than it is the sex. I don't base my relationships around the sexuality of them. I find that sex comes later. Or it doesn't. I have some really loving deep connections with people who I don't make love with sexually.
To me it seems that your gal is just being a little wild and desperate at the moment. Honestly, that's okay, if that's the stage she's at in her life, it's important for her to have this experience. However, it seems to me that you are wanting something different. You might ask yourself if this relationship has a future, if you can grow together. Weigh the pros and cons. Maybe give it a couple of months, since it's new, to see how things go.
And remember, don't ever accept second best. It's okay to be discerning. You may think that you are bad at romance and meeting people, but there's a lid to every pot; and there's probably someone or someones out there who are looking for someone just like you.
Both of the men in my life are very shy people, and they both are awkward about meeting new folks. I approached both of them, rather than the other way around, and I am SO glad that I did. I'm a firm believer in still waters run deep, and it definitely did with these two. I'm sure that there's someone out there who will appreciate you for you, just the way you are. Unfortunately to me, it sounds as though your girlfriend may just have a different way of being in mind right now. Good luck!!