EXCLUSIVE: Shannen Doherty Breaks Down Over Cancer Battle: 'I Don't Look Past Today'

This video is unavailable because we were unable to load a message from our sponsors.

If you are using ad-blocking software, please disable it and reload the page.

Shannen Doherty's cancer fight has taken a heartbreaking toll on her.

The 45-year-old actress revealed last August that she was diagnosed with breast cancer in February 2015, and in an exclusive new interview with ET, Doherty cries when speaking about her intense battle.

"It's hard," she acknowledges to ET's Jennifer Peros, shedding tears. "I started thinking that I wasn't going to live... I don't look past today."

"I don't know if I'll ever look like me again," she adds, wearing a scarf over her head.

WATCH: Shannen Doherty Chronicles Shaving Her Head Amid Breast Cancer Battle

In the emotional sit-down, Doherty reveals a devastating setback in her diagnosis for the first time.

"I want to believe that if I fight, I'm gonna be OK," she says.

Doherty's exclusive interview airs on Monday, only on ET.

On Wednesday, ET chatted with her former Beverly Hills, 90210 co-star, Jason Priestley, who shared his love and well-wishes for his on-screen sister. Priestley was especially moved by Doherty's decision to share pictures of her shaving her head on Instagram.

"What she did the other day, shaving her head and posting the pictures and everything, that was incredibly brave," Priestley, 46, said. "Shannen's a strong kid and a tough fighter, and I know that she's gonna come through this fight and she's gonna be OK."

WATCH: Shannen Doherty Opens Up About Cancer Battle and Her Career -- People Think I'm 'Unable to Perform'

Fellow Beverly Hills, 90210 actress Tiffani Thiessen also shared kind words.

"I wish her all the best, of course," Thiessen told ET.

Watch below: