Try thinking about it this way. What if everything was exactly the same except you were single. What would you do and think then? It's ok to be falling for someone when you thought it was just going to be casual, it happens to people both poly and mono all the time.
The fact that she says she has a hard time missing you makes me think she won't freak out if you let her know that you might be crushing out on her just a little. Let her know that you really like her and ask if she'd like to go out for coffee sometime (you don't even have to call it a date).
The major players. Me, 30ish bi female. Gia, girlfriend of 4+ years. Clay, boyfriend/dom. Davis, ex/friend/"it's complicated." Eddie, roommate & fwb.
The supporting cast. Eric, Gia's husband. Bee, Gia and Eric's toddler. Dexter, Gia's lover. Helen, Eric's lover. Izzy and Nikki, Clay's partners. Liam, Eddie's husband.