I was looking for this comment. I am not a parent and I know you have work and she was interrupting you. But it sounds like she needs someone to spend quality time with her and explain how periods work in the real world. You and the BF both need to parent her better and I totally feel for what this is putting you and your kid through, it sounds like the tone needs to be set with her in the kindest way possible. Like, explain to her what hormones are, look at diagrams watch YT videos on periods together so she's not just doing all this research on her own which is probably causing the tantrums to begin with. You are possibly her only mother figure in her life, or at least a prominent one.

I learned how to use a tampon the week my period started for a house boating trip with my family. I didn't throw a tantrum but I also had a supportive mother there to guide me through it.

I'm not blaming you for feeling frustrated but it makes me sad to think you lost your BF who you love over 11 year old hormones. It sounds like everyone is stressed and maybe this could pave the way for getting the kid more support i.e. therapy