I agree that middle school – really through 9th grade – was a tough time for friendships among girls. I don’t really have horror stories, but my daughter did what amounted to a complete replacement of her friendship circle around 7th grade. Fairly minor differences of opinion, mainly about boys and their importance in the world, and what was appropriate to do with them, caused some major rifts. By the middle of 11th grade, even though my daughter had left her original school by then, she had largely made up with the friends she had lost in 7th grade, while at the same time making at least one completely new friend that became her closest friend of all.
She was visiting a few weeks ago, with her boyfriend who was here on business. One day while the boyfriend was working the two of us had lunch with a woman whom she (and I) first met when they were in pre-school together for a year at 3 years old, then they were inseparable best-best friends from 1st grade - 6th grade, although by the end the relationship was so inbred and toxic that everyone agreed they had to be separated. They barely spoke for three years, then slowly reconciled, and in 12th grade actually published a zine together. They are – and always have been – totally different people, with barely anything in common. It doesn’t matter. They remain friends, and have learned how to appreciate and accommodate one another. It was a real treat to spend time with the two of them together.