Is it normal to feel this way?

Alright, I know I'm probably going to sound too possessive or protective or overdramatic or what-have-you, but I just need to know if there's something wrong about the intensity of how I'm feeling right now.

So let's start with the story. I had three friends, but one of them hurt me very badly, to the point that I had cut myself just to cope and even thought of killing myself (and I'm usually not the type to do something like that). She talked about me and other people behind my back, picked on my weaknesses, openly admits that she enjoys abusing people who like her and watching me look miserable, and then when I confront her about it, she tells me that the "voices in her head" told her to do those things (which, by the way, only suddenly appeared one day and find flaws in everyone except her crushes). In the midst of it, I (regretfully) tried to seek help from the rest of my friends since I didn't trust our school guidance counselors and my mom shrugged it off, and naturally it made it to her and she kept using that against me as a reason for her to lose her trust in me. I felt like utter crap for doing that.. We had a falling out and when I tried to talk to her about it a month later, she says that her dad wants to sue me, get physical with me, and that he says my mom wouldn't stand a chance (all hit me hard, because she knows I had an abusive father). She was also known to make up stories from having writing impairments to seeing Slender Man while on a picnic with her family. She also talked to one of my less close friends about how I was stupid for cutting myself about "petty things".

Anyway, my other two friends know everything that happened, and were with me through all of my breakdowns about her. But.. Although we don't hang out with her as a group anymore, they still talk to her casually like nothing happened, and sometimes they even hug and dance while she ignores me. It hurts me because she has hurt me so freakin' badly and both of my friends know that, yet..

Maybe I'm just being too possessive over them, after all, they can be friends with whoever they want right?.. but they know she likes to lie and exaggerate, and that she blames me for everything, so sometimes I fear that they're on her side talking about me.. I don't know. I know I should trust them; they both say they love me deeply and that I'm the best friend they could ever have. We've all been vulnerable with each other, shared dreams, helped each other with most other problems. That's why I try to act normal and happy when that stuff happens but really I'm dying inside.

Sigh, well, I just want to know if I'm overreacting, and if I am, what I can do about it. Sorry for the long story.