I can see how that'd be really tough. Try to focus on the fact that 1) even if the other gf weren't in the picture you still wouldn't be sharing this time with him, and 2) them getting closer (which sharing experiences can do) doesn't mean you two will be further apart. "Evening the score" by having her be separated from him the way you are wouldn't bring anything good into your life. In fact, the stress of missing *both* his partners could have just made him less emotionally available to you (some people close off when they're too stressed). She is not the enemy or the thing keeping you and him apart -- the distance is.
All that said, let him know you're feeling extra vulnerable, and ask him to try hard to stay in touch. That's certainly not too much to ask. Great resources in dealing with jealousy at www.morethantwo.com
Ps -- Even though you're not sure yet if you yourself are poly, I would encourage you to keep going on dates, especially now. Maybe it'll turn out you are, but either way it'll be a helpful distraction so you don't go crazy from missing him.
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.