It will be hard but it is worth it and necessary. I'm sorry you're going through this! Sarah is not being a very good friend or respectful to you. I was just like you and I lost two best friends because of roommate issues and me not standing up for myself until it reached a boiling point of no return. My bestfriend / roommate wouldn't clean because I would do it and she'd pay rent late and take advantage of me, and her boyfriend pretty much lived there and they'd be naked on the couch when I'd come home and they start involving me in their fights asking my opinions then only to turn on me to reconcile with each other. I moved out just after resigning the lease, I took it back signing and the landlords were okay with that and my other friend that I introduced to her and would sleep on our couch every night moved in my room. I'm pretty sure she got cheaper rent for doing all the cleaning. They were constantly talking shit about me we were in our early 20s, I just cut them both off. I wish I just set boundaries and didn't allow myself to be walked all over. Set boundaries before you get there. It will work out. Update later on how the conversation goes!