I read this book at least six years ago. I only know small details of
the plot. It is fiction and I believe it was written by a woman.

A woman finds out her boyfriend is cheating on her with another woman.
She ends up breaking into the womans apt (she must have been out of
town) and living her life as this woman.  Wears her clothes, uses her
name etc.

I really really want to find this book. THanks

This sounds a bit like "SWF Seeks Same," by John Lutz.

"Things begin to fall apart for computer consultant Allie Jones when
she discovers that her live-in lover, investment broker Sam Rawson,
has strayed. After throwing him out, she needs a new roommate to help
pay the rent. A discreet newspaper advertisement brings in Hedra
Carlson, a mousy young woman who quickly begins to idolize and imitate
her. A quiet air of menace develops, enhanced by Lutz's simple, direct
prose. Is Hedra literally stealing Allie's life?"

http://www.amazon.com/exec/obidos/tg/detail/-/0312051654

thanks pinkfreud but that is the movie Single White Female. (must have
been a book before it was a movie I guess huh?) Thanks so much for the
comment though. Man I was hoping you had nailed it.