I've been hoping my girlfriend and I will 'graduate' up to monogamy

When I met my girlfriend she was "experimenting with non-monogamy" (her words) I held onto hope that the experiment would end someday. I started as one of her 3 partners, now I'm her boyfriend, and they aren't. We do more than have sex; we've met each other's friends and family. But she still sees them occasionally. They are poly as well, so I know that I am the only 1 truly committed to her. Going from partner to boyfriend is clearly progress in the right direction. Will I ever be enough?

No, no, nope. I am sorry, letter-writer, but you're going about this in a way that's wrong-headed and will ultimately cause you a lot of pain.

You went into the relationship from a place of denial, hoping that your partner wasn't serious when she told you something about herself. You latched onto the word "experiment" and told yourself that it would end. But that's not what she meant, it seems. Sometimes "experiment" means temporary - sometimes it means "checking out a hypothesis." If her hypothesis was "I'll be happy in a non-monogamous relationship," and the experiment showed her hypothesis to be true, then maybe the experimental phase is over, and she's now in "acting on the information gathered during the experiment - living my life as a person who knows she's into non-monogamy." Your first assumption was that the non-monogamy was a temporary phase, and this assumption was wrong - you're acting on an incorrect assumption, and that won't end well. 

You also have this idea that them being polyamorous means that they are not "truly committed to her," and that means that your feelings for her are somehow deeper or different. That is not true, it is not how polyamory works. Your second assumption is that these other relationships she has are lesser, less committed, less threatening, less real. That is also an incorrect assumption that you're working on. You're also assuming that you are "not enough" for your girlfriend, and are interpreting all of her choices through a lens of projected monogamy that is warping how you understand what's going on.

You also have a sense that your relationship is going in "a direction" toward monogamy, which is "the right" direction. That is also not how these things work. You do not grit your teeth through the 9 stages of non-monogamy, then present your stamp-card for Level 10, Monogamous Boyfriend. Your girlfriend probably doesn't see it this way - you two need to have a serious talk about this. Assuming that this is just a natural, inherent progression in your relationship is a dangerously incorrect assumption, and people will get hurt if you continue on this path. 

It sounds like this is not a healthy relationship for you to be in. It's build on assumptions, denial, and expectations that show no indication of being fulfilled. Your first step is to have a serious talk with your girlfriend about her perspective - and to really listen and understand what she's saying, not filter, interpret, argue, or push her into concessions. ("Could you maybe see yourself with just me, someday?" - If she says yes, she might be thinking, I guess I can't predict the future, there's always 'a chance' of whatever but you might be hearing more of a promise.) After that conversation happens, you'll have a better picture of whether this is a relationship you should stay in, or whether your hopes for the future are just not on the table.