First of all, you're a good guy. I have to tell you that because entirely too many guys your age get bitter and sad over things like this and turn into rotten bastards. What you are, right now, is a good guy with standards and you're not willing to compromise them. That is fine. Don't compromise them. Don't doubt who you are. And don't let your experiences turn you bitter and sad. Someday, a very nice person will thank you for growing through your pain.
I'm sure your girlfriend is a good person, too. I echo the sentiment that she may not be right for you, but that doesn't make her bad. I question the ethics of keeping other partners a secret from you, but then I remember being nineteen and having no words for the vague poly longings I had. If she has the words, well, she's one-up on me but still nineteen. Society still treats nineteen like an adolescent; as a result, young adults up to about the age of twenty-two have been merged back into that teenage territory.
So I did some stupid shit. One thing I did not do, however, was insist upon staying with someone who expressed fundamental incompatibilities with me. I think that's a talk you two have to have, now, not down any line.
I don't think she is a gold-digger, either. I think she's letting you pay for nice things, yes, and it's awfully nice to experience those. I also think that if you two find a way to make this work, she should learn how to love things that don't cost money. That's not a poly thing. That's a living-within-one's-means thing. That's an adult thing. There is [cough] a significant income gap between myself and CdM, and we did have The Money Talk early. I am a cheap date. Literally.
I am happy just cooking with him, or doing an inexpensive night out. Do I also love it when he shops with me? Sure -- but on special occasions, like birthdays. And I love to shop for him as well as being shopped for. It's just as blissful having little adventures as big ones.
Mind you, I learned the value of a dollar the hard way: by spending a few too many!
I wish you the best of luck. Do feel free to contact me in PM or here, and I keep a blog thread.