Maura Tierney on Her Breast Cancer Diagnosis

ER actress Maura Tierney was diagnosed with breast cancer in 2009, and now she's speaking out about her once private battle with the disease -- including the shocking moment she learned of her diagnosis.

"I remember thinking, 'I'm so young, this can't be happening,'" she tells People. "It was an odd moment."

Tierney is now finished with treatment, which included undergoing a mastectomy and three months of regular chemotherapy.

"It's not fun. You don't feel well," Tierney says about the grueling process. "[But] "if you take care of yourself and exercise, it helps. Exercising is extremely important."

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And though getting cancer was definitely a monumental event in her life, the actress isn't letting the disease affect her free-spirited way of living.

"I think I always kind of lived in the moment. I don't think [cancer] changed me that way," she shares.

Though there's one positive aspect that came out of the situation -- a closer relationship with her loved ones.

"But I spend a lot more time with my family now – that's one solid difference," she says.

Related: Kristin Chenoweth & Maura Tierney Join 'The Good Wife'

Next up for Tierney is a guest-starring role on The Good Wife this fall. She's also now the spokeswoman for Amgen's Chemotherapy: Myths or Facts campaign, where she's sharing her own experience with cancer and is encouraging patients to speak openly with their doctors to fully understand their treatment.