Am I going to lose a best friend?

Instead of confronting her, maybe sit down and have a discussion? You aren't in the wrong here, your feelings are totally justified. Whilst it would be a shame to lose friend B, you don't know what will happen until you all try and sort it out. Good luck!

You are not in the wrong.
It seems like B is going to lose every other friend she has- if she prefers to spend her time being best buddies with a vicious bully like A.
That is her choice and her loss.
Life is too short to waste on horrible troublemakers and fake friends.

A is probably up to her old nasty tricks again..
Enjoying herself telling B nasty lies about everyone she dislikes, trying to start fights between you all and probably asking B for information or to spread her vicious rumours too.
But there is nothing that you can do except protect your own mental health and physical safety from the pair of them (A and B).

There used to be four of us as a group of best friends. Friend A, B, C and me.

Friend A would bully me and C and be horrible to us. She ruined my initial experience of school and forced me into a depressive state.

She even told me she didn’t want to be friends with me anymore and that I wouldn’t be hard to forget...

One thing led to another and we slowly drifted apart, but I never forgave her, really.

So we cut things off and found friend D who’s now one of my closest friends. The problem is, A and B are suddenly now best friends again and it makes the rest of us uncomfortable.

It wasn’t bad at first, but now A’s always around B, and B has started spending more and more time with her instead of us. It’s almost like she has a preference.

And it makes the rest of us really sad because she was so horrible to us, constantly made us cry but because B never experienced it, she completely glossed over it all.

We want to confront B and tell her how we feel but I’m not sure what our actual goal is. Worst, comes to worst, our 5 year long friendship will end because of this and I wouldn’t be able to cope if that happens.

What should I do? Also, am I in the wrong here?

Also, sorry this is so long. I’d make a TL;DR but I feel like I won’t be able to properly condense this without missing out important details.

I'm really sorry to hear of this. It sounds like there is a power dynamic within your friendship group and this leads to exploitation. I try to find groups that are based on equality. It's not right that A treats you the way you do and then you have to be worried if the friendship ends. You deserve fair friendship based on mutual respect not power and social skills. It doesn't sound like you are wrong here. You are a person worthy of equality and friendship there's always nice people out there. I hope this helped

Thank you so much. I’ll consider this as I head into year 12 and am looking for new friends!

We did try to talk to her but we didn’t really say what was on our minds, and before you knew it, everything turned awkward. Really awkward. So, we’ll just have to see what happens, I suppose.