OP, I was your daughter all through primary and until around year 9/10 (age 14-15). My “best friend” was controlling, constantly talked down to me, insulted all of my interests, made constant jibes about my looks and intelligence, and slowly eroded my confidence over the years. She would get involved in any other friendships I had, triangulate, and ruin them. Every single club or activity I did outside school, she’d join and ruin for me.
When we started secondary school, she was initially placed into a different form to me and I was actually really happy and confident for those first few weeks, making new friends and feeling popular for the first time ever. Then she had an epic tantrum, and her mum demanded she was moved into the same form as me, despite there being a group of lovely girls in her form who had been really kind and tried to befriend her. School was never the same after that, she meddled in the burgeoning friendships I’d made, turned them against me and befriended one of my newest friends and used this friendship to treat me badly. I’m not sure exactly what happened, perhaps it was just a natural maturing but at around age 15 I made a new best friend who saw right through and hated my BF and so couldn’t be “stolen”, and she finally lost her power over me. I just no longer cared what she thought.
Reading your posts, I feel really sad for your daughter as I know exactly what she is going through and she’s probably not quite mature enough yet to remove herself from this friendship on her own. I never confided in anyone how the friendship affected me, and I’m not sure my mum even knew. But you know, and you can do things to help her.
Is she in the same form as this girl, if so, can you request that she be moved? Are there any other nice kids she could become friends with away from the influence of BFF?
Do you have any friends with children of a similar age, who go to different schools, that you could potentially meet up with?
How does BFF find out about these clubs your DD signs up for, does your DD tell her? If so, I’m not sure how you can stop her from doing this, you could try and have a chat to her about keeping any new hobbies to herself if she is amenable. But maybe try and find a club that she is interested in and speak to the leader and explain the history of this toxic friendship, that BFF has a habit of following DD to new clubs in order to ruin it for her, and you want her to have an opportunity to explore interests and grow confidence away from her. I have no idea if it’s even a thing a club can do, but you could ask if they can effectively “ban” BFF from joining, citing numbers/lack of space.
Unfortunately your DD won’t move on from this friendship until she realises that she can have other healthy, happy friendships away from BFF. Hopefully this will happen sooner rather than later, but my toxic friendship has affected me for my entire life. Even now in my 40s, I battle with feeling of inadequacy in friendships and believe deep down that people don’t actually like me.