Oh sweetie, I’m sorry you’re in such a position. When you’re in a relationship of any kind, mono or poly, it’s perfectly ok to communicate when you don’t think you’re getting enough time and attention. You say you worry about “nagging” and I completely sympathize. But you can be proud of the love that inspires you to want more of this person than you are getting. You can be doubly proud of communicating your needs. One way to get around the feeling of naggings is to phrase your needs simply and directly, maybe writing it out, like you did here.
In your writing I can see your pain and your good will. It seems like you’re entangled with a couple of somewhat chaotic individuals. As a secondary in a relationship with someone 11 years my senior, some of the best advice I got on this forum was to go do things that interest you. “Well, HE interests me and THAT’S what I want to do,” was my response at first, but it really is good advice. In a chaotic love-space that can get foggy with emotion, it’s important to use the time apart you’ve got to cultivate a relationship with yourself
Wishing you the best of luck.