SOLVED. 90s YA about a girl named Cath/Catherine who lives with single mom and gets into a dysfunctional relationship with a boy named Tony. [s]

[Rebecca]

This has been driving me bonkers for months. I read/loved this paperback as a teen and can't remember or figure out the name of it. Things I remember:

It was published in the early 90s or the late 80s.
The main character was named Cath, but the love interest called her Catherine.
She drove a Volkswagen Beetle I think?
She lived with her mom, who was studying to be a lawyer.
Most chapters began with a note that Cath and her mom left each other on the fridge.
She had one best friend who I can't remember the name of...one of the side stories was the friend deciding to have sex with her boyfriend or not.
She loved writing and got along well with her English teacher.
The mom started a relationship with a lawyer named Mr. Donnelly or something similar.
Cath's love interest was named Tony.
Tony rode a motorcycle.
Tony had a bad relationship with his father and was often unhappy. He sometimes drank to cope.
They went to a school dance at one point, and liked to hang out in a park.
Cath didn't know how to help him, which caused problems.
They ended up breaking up because Tony was drunk driving in her car and he made Cath take the fall for it.

If someone can figure out what this book is, I'll be eternally grateful!

[Tab]

One Step Short

[Rebecca]

OMG thank you! That's not the exact book but it is the right author, and it brought me to the book I was thinking of, which is this: Maybe By Then I'll Understand