Might this be Eyes in the Mirror (2011) by Julia Mayer...?
I always thought it would be cool to escape into another world. I never believed I'd find one in my reflection. But there I was, falling through the mirror into a parallel life-Samara's life. And she needed me. The cutting, the dead mom, no friends...She was hurting, and I knew it was up to me to fix it. She needed me to fix her.
She'll thank me...
Samara I never had a friend until I met Dee, at least not a real friend. But then she's my reflection, so maybe I'm just crazy. When she suggested we switch places, it seemed like the perfect answer.
So I let her live in my world, and I lived in hers.
With her mom, her boyfriend, her friends-her perfect life...I don't belong here.
But how can I go back after what she's done?
Two girls, one reflection, and a startling discovery about what really lies beyond the bathroom mirror...
From Kirkus Reviews:
Stoned, Dee’s attractive male friend Jamie suggests the existence of an alternate universe, and later the 16-year-old finds it by stepping through a mirror.
On the other side, she encounters Samara, her alter ego. Where Dee is a good student and rarely steps off the straight and narrow, Samara, an unhappy loner since her mother committed suicide three years before, savors cutting herself, an act that’s vividly depicted a few times throughout the narrative. Told in these two girls’ voices in alternate chapters, their stories interweave when they inexplicably decide to swap places for a day or two. Samara encourages Jamie’s sexual advances, and Dee tells Samara’s father about her cutting, leaving both teens with new issues when they return to their own worlds. While Samara must deal with therapy, Dee faces greater trials. Though these push her off her college-track, she adjusts with remarkably minor angst. The quality of writing is uneven, too often telling rather than showing and never managing to generate sufficient suspense to move the tale forward. Both Dee and Samara are described in detail, yet remain stock characters—good teen and troubled teen. Others are more superficially developed. While the concept is intriguing, the paranormal aspect is overshadowed by the less compelling reality-fiction component.
Ultimately, this average debut effort never rises above the masses in either overcrowded genre.