I found out my bestfriend has been sabotaging all my other friendships
I (17 F) found out my best friend Zelda (17 F) has been sabotaging my friendships, and I don't know how to handle it. Before I start sorry about the length and anything I do wrong, bear with me, this is my first post. All names are fake.
For a backstory, I've known Zelda since our Freshman year of high school. We met through mutual friends but didn't become close till Sophomore year due to doing theater together. We made a friend group that didn't last long, but we still talked sometimes in the halls. The next year (this school year) we created another friend group that has been thriving, and we became super close. In this group there is Lydia (17 F) and Mavis (17 F). I've known Lydia since Freshman year too, but we've always been closer with each other than Zelda. I met Mavis Sophomore year in theater, but we didn't become super close till this year.
In the beginning of the school year Lydia got into a relationship with one of my close friends who I used to like from middle school. Key word used to who we'll call Brian (17 M), and Zelda did not like it. This relationship started after a traumatic friendship breakup and a toxic relationship. So she was recovering from all of that. Brian joined our theaters tech, and Lydia got a lead in our musical. Lydia was under a lot of stress from this and would talk to me and Zelda about it often, which sometimes came across as both bragging and complaining. This irritated us and Zelda would talk to me about it often which made me upset with Lydia. This was the first red flag, but I didn't see it because she'd always been like this with her friends.
Since Lydia and Brians relationship was so new, they were in the "honeymoon stage". During this time Lydia would talk about Brian constantly. I'm talking she would bring him up in every conversation, no matter how unrelated it was. After some time, this made me upset and very annoyed with her. I felt that he had become our entire friendship and tension built up because of that on all sides. Zelda and I would talk about how much Lydia was talking about Brian, and how it upset us. At first, it was rare that we would discuss this, but as time went on it was brought up more. To be honest it's hard to tell who was bringing it up seeing as our conversations would naturally drift to that topic. During all of this Mavis decided to take a break from acting and joined tech, so we rarely saw her outside of our weekly hangouts. Meaning she didn't see what Zelda and I saw/heard with Lydia. Later Mavis went to me and Zelda about how she also was upset with Lydia about Brian and how she acted as a lead.
During the show I started dating a guy in our group Chud (18 M). Most people in the group weren't a fan of our relationship because they thought when we broke up it would completely destroy the friend group, which I constantly reassured them wouldn't happen. I have recently found out that Zelda was so mad about both us getting together and breaking up and she would talk to Mavis and Lydia about it often.
The night before our first show our theater has some fun traditions that we do that involves driving. Since Zelda was mad at Lydia, she refused to drive her around until Lydia decided she'd rather not participate than feel lonely. Which then Zelda agreed to drive her.
When the production was in full force (towards the end of rehearsals) yet another friend group was created by another friend. Which included Lydia, Mavis, Zelda, Cate (17 F), Amy (17 F), and a few others. Over the course of a few weeks Zelda had completely replaced our last group with the new one and had obviously replaced the "girl group" we had with herself Cate and Amy, which really stung to see. Mavis even called me crying about it once which is when I truly started to notice. And we even found out they had a secret friend group that Zelda lied about. And in this new group Zelda continued to talk about Lydia causing Cate and Amy to dislike her. So much so that Amy openly talked about disliking her to me a few weeks ago, which I was so confused about.
Onto the problem, Zelda has been upset with Lydia, so I asked her what was going on. Me and Lydia have been better than ever for about a month, and we have both moved on from the previous conflicts and Lydia has apologized to everyone. Zelda told me that Lydia didn't seem like a friend, and she probably wasn't going to mend the relationship until Lydia and Brian broke up. Mind you Lydia hasn't overly talked about Brian in roughly a month and took full accountability for her side of what happened. So, I called Lydia to talk about it, since I told her I'd ask why Zelda was acting off still. During my conversation with Zelda, she told me that Cate was telling people that Lydia and Brian were making out in the back of her car on their way to Dairy Queen after Brians prom, which did not happen. Zelda just seems to have a problem with Lydia having a boyfriend in general for no apparent reason. Even though Brian was nothing but nice to everyone.
After I told Lydia everything, we had the biggest debrief of our lives. We told each other everything. Everything that we've said about each other, what others have said, secret group chats that were made, just everything. A quick-ish summary of the 2-hour conversation is Zelda has talked shit about every person in all our friend groups and lied to that persons face about it. For example, when Chud and I started dating, Zelda talked to everyone about how she was mad at me for it. And she even went as far as to create a group chat with her, Lydia, Mavis, and one of our guy friends in the group where they all bet on how long the relationship would last (Lydia won). Zelda told me that she wasn't upset about it and was just happy for me and kept telling Lydia she was being crazy for how mad she was. But she told Lydia that she was pissed. Another example (a worse example) Lydia realized after she and Brian started dating how much I liked him and he liked me, back in middle school which caused her to over think. She decided to confide in Zelda about this, who fed into her insecurities. She would make up lies about me and Brian's interactions (she said we were openly flirting when Lydia wasn't around) during rehearsals and would tell Lydia about it, which made her jealous. And the "flirting" she was talking about was just me cussing at him and stuff like that. A few months after this happened Zelda told me about it but instead said that she told Lydia about how she's just overthinking and being crazy anio8kd how I would never do that to her. And there have been other instances of this with me and Lydia, and my other friends. She constantly lies to us to cause drama or to get out of a situation she doesn't feel like dealing with. We also realized that Zelda was using Lydias relationship. When Zelda had a crush on a guy, she would make Lydia have Brian talk to him to get information. And that was the only time Zelda has every "liked" Lydias relationship.
Basically, in every conflict I've had with this friend group she has caused me to be riled up by talking about it so much, and she would do the same to the other side by just saying different things. And I know for a fact if she had not been there, I wouldn't have been anywhere near as mad as I was. She just loves to talk shit. She seems to always be in the center of conflicts with anyone. It feels to me as though a lot of it is jealousy but I can't be for sure. Zelda also just makes sure she treks everyone what happened before the other person realizes there's a conflict. This has been a pattern outsiders have noticed too.
I need advice on how I should handle this situation. Like I don't know if I should talk to her, or leave it be, or what. And before anyone says it, there is no world in which I can cut her off since it's about to be our Senior year and we both still do theater together AND we're in several friend groups together. So reddit, what do I do?