This Lullabye by Sarah Dessen (Used Book)

4.02 stars on Goodreads

The science of breaking up...

Remy always knows when to give a guy "the speech" - right after the initial romantic rush, but before anything gets too serious. She's had her fair share of boyfriends, and she's learned all there is to learn from her mother, who's currently working on husband number five. So why is it that's Remy can't seem to dump Dexter? It can't be his name. It can't be that he's messy and disorganized. And it certainly isn't that he's a musician - just like Remy's father, a man she never knew because he left before she was born. Could it be that Remy's romantic rules to live by don't apply anymore?