Welcome to our forum.
When you finish the Ethical Slut, pick up a copy of "Opening Up
," by Tristan Taormino. It may help you navigate this confusing swirl of non-monogamous possibilities, including how to handle a polyamorous/monogamous relationship.
On this site, I would do some searches and tag searches for mono/poly and seeing what comes up. Things are probably fine if your girlfriend just wants to be monogamous while letting you be polyamorous, but if she can't stand being with you when you're poly, then you have some hard decisions to make.
You should do quite a bit of reading on this site, along with posting your thoughts and questions, and try to decide, through that process, what it is you want and/or need in life. Is polyamory the only way for you, that will lead you to true happiness? Are one-night stands good enough? Don't hurry this decision.
I would actually not talk a lot about this decisionmaking process with your girlfriend unless she asks. Something about non-monogamy is upsetting her, so give her a rest from that upset for a little while. Raise the subject again when you feel sure of what you want (as far as relationship style is concerned). In the meantime, work on your relationship issues with her, and ask her what you can do to help her feel more reassured and happy with the relationship.
If it comes downs to a dealbreaker, e.g., you absolutely must pursue a poly lifestyle, and she absolutely can't deal with that in a relationship with you, then you may have to talk about breaking up. But that hasn't come up yet, and it may not come up in a long time, if ever. Let's cross that bridge when we get to it.
Maybe she thought she could do poly in theory, but in practice, her heart is saying no. That happens to people sometimes. Sometimes they can work through those feelings, sometimes they can't. Depends how hard-wired-for-monogamy your girlfriend is.
I hope Polyamory.com can be of some help.