Well, yeah, you're definitely in the right place... you just described polyamory.
You say you don't know how to voice your feelings -- what's so confusing about "I think I'm falling for this person"? I mean, you call her your girlfriend, in what world should "girlfriend" not include feelings, alone time, dates, maybe love or commitment? That would just make someone a friend-with-benefits, right?
and do some reading. You're gonna be ok.
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.