IMPORTANT...Friend treats me like crap!!!?

K so I have this friend. I'll call her Sarah. I have known sarah for almost a year now and we definitely have had good times together. Though, lately shes been treating me like crap. For the record, I see myself as a really nice person and I NEVER get into fights or arguments with anyone. I kinda keep my feelings to myself and I can be seen as a pushover who never sticks up for herself. So, I have never done anything bar to Sarah but she is constantly putting me down. For example, if she wants to hang out and it's late or I just don't feel up to it, she would say things like you're the worst friend ever and i hate you and you're horrible and stuff like that. It really bothers me. So today she was doing the same thing and blackmailing me with something I Lost saying she might have it and it's really important worth like $500 and she wouldn't give it to me if I didn't do something! So i said i was fed up and wasn't going to hang out with her and my other friend on Friday. This is the first time I have ever stood up for self and she didn't care about my feelings at all. My other friend took her side and they both ganged up on me saying I ruined all of their plans! But not once did she apologize or ask if I was okay when I was crying in the phone because they were making me feel so bad about myself. All they cared about was our plans on friday and how they're "ruined" now. I have seen both of them in a new light seeing as they didn't even care about the fact that i was crying and feeling bad. How can i be sure to not breakdown and apologize for something Im not sorry for. What should I do when face them tomorrow?!