I read this book sometime between 2004 and 2008. It's a psychological thriller about an unhinged woman who wants to remain stuck in her adolescence. She has two close friends who she was at school with (pretty sure it was a boarding school). Friend A is rich and Friend B is poor- she is still friends with both of them but they are no longer friends with each other, as when they were still at school she framed Friend B for stealing Friend A's bracelet, as she wanted to keep both friends for herself without having to share them with each other.
The only other two details I can remember are that she still sometimes wears her school uniform as an adult (only when alone in her house) and that, near the end, she somehow causes Friend B's girlfriend (who is allergic to bees) to be stung by a bee, as she is jealous that she is no longer the closest person to Friend B.
I would be so grateful if anyone could help me identify this book!