It seems like maybe she's ready to break up and is using poly as a way to do it without having to admit to you (or maybe even to herself!) what's going on. I'm sorry to be so blunt, but considering the way this was introduced to you, you're handling it amazingly, whereas she doesn't seem to be willing to consider your needs at any point. I would ask her point blank if she's not into the relationship any more, maybe even press her on the topic. If she insists that she's still invested and you want to keep trying, give her some things about NRE to read and hold her to future promises about alone time (not sex, necessarily, most people don't do well under that sort of pressure, but making time just for the two of you). Poly should never be an excuse for neglecting any of your partners!
Me, 30ish bi female, been doing solo poly for roughly 5 years. Gia, Clay, and Pike, my partners. Davis, ex/friend/"it's complicated." Eric, Gia's husband. Bee, Gia and Eric's toddler.