[MFF] Strained Roommate Situation

Background:
My gf [F22] and I [M28] started dating roughly a year and a half ago and she’s been a great partner and guide to helping me be more comfortable and confident with myself sexually. My gf has had the same closenit group of friends and we ended up moving in with one of her friends (we’ll call her Sally [F21]) about a year ago (6 months into the relationship). The apartment that we moved into consists of two bedrooms that connect to a shared living room/kitchen area with a shared bathroom on the other end of the living room.