She wants our communication to feel light and fun, and I find it hard to not have a serious conversation around it...
Not about what either of you wants.
It's about what the healthy polyship needs.
If that is the shared overarching mission -- to live and love in an ethical, healthy polyshipping dynamic of some kind?
Maybe you can talk some of your stuff out with a counselor to organize it down to Cliff Notes so she's not overwhelmed and you still get to air out whatever it is you need to air out and organize your thoughts. Then BOTH get what the want/need while still serving the polyship at large's needs.
She has had many more lovers than me and lived a pretty free spirited sex life outside of the times she was in relationships(and even within them, which she didn't like having to be sneaky). Poly is new to me (I was much more a serial monogamist) and I need to do much more work on it to feel comfortable.
Are you needing reassurance in the dept that she isn't going to be sneaking around/cheating on YOU? If so, could ask her for it.