Can you name the book?

I read this story years ago, and I’ve forgotten the name. I tried searching for it on Google, but couldn’t find it so I’m here asking if anyone could name the book. The main guys name is Sebastian. Everybody in this story is super rich. I remember in the first chapter, the girl is headstrong and hits Sebastian on his head with a baseball bat. This causes her to be suspended from the school and she moves away. The story continues as they are grown-up and this is the first time that she comes back home. She goes to a party with her half sisters where she sleeps with an unknown guy and loses a diamond ring. The next day she finds out that the guy is Sebastian. And then later, they start fake dating to make his ex jealous, which later turns into real feelings. They get together, and I only remember that at the end, they break up and then there’s a book two.