You're in a sticky situation, to be sure.
What would I do in that situation? Lay your cards on the table. Trying to hide your feelings is a life-killer. How can you live with these 2, share lives, meals, television shows (I assume) cuddling on the couch, stories of your day, sexy times, while hiding your romantic feelings for H?
You started out as FWBs and now you're in love. Possibly jealous gf there in your face... Be prepared to move out if she suspects or confronts you with feeling "too much" for her bf.
You're playing with fire and you're getting burned. You're young, I am sure, attractive. Might be better to look around and find a local bf/gf, either before moving out, or after.
BTW, you are definitely NOT a unicorn. A Unicorn is a mythological beast that couples search for to "share" equally. What you've got is a sort of partially romantic triad. You're not really anyone's girlfriend, if you can't even talk about loving emotions.
Love withers under constraint; its very essence is liberty. It is compatible neither with envy, jealousy or fear. It is there most pure, perfect and unlimited when its votaries live in confidence, equality and unreserve. -- Shelley
Mags (poly, F, 61) loving miss pixi (poly, F, 39) since January 2009, living together since 2013
also loving Punk (42, M) since Oct 2015 (he has recently downgraded us to friends)
"Master," (mono, 34), miss pixi's Dom for 2 years