Maura Tierney: Cancer Battle Made Me Realize the Importance of Family

When Maura Tierney was diagnosed with an early stage of breast cancer in 2009, she kept the details of her treatment relatively private – until now.

“I remember thinking, ‘I’m so young, this can’t be happening,’ ” Tierney, 47, tells PEOPLE in this week’s issue, on newsstands now. “It was an odd moment.”

After undergoing a skin-sparing mastectomy – a procedure that leaves the skin intact for reconstructive surgery – Tierney was shocked to learn that she also required three months of regular chemotherapy.

“It’s not fun. You don’t feel well,” says the actress, who now works as a spokeswoman for Amgen’s Chemotherapy: Myths or Facts campaign. But “if you take care of yourself and exercise, it helps. Exercising is extremely important.”

Now finished with treatment, Tierney – who will guest-star on The Good Wife this fall – gets a yearly MRI and mammogram and is focused on staying strong.

“I think I always kind of lived in the moment. I don’t think [cancer] changed me that way,” she says. “But I spend a lot more time with my family now – that’s one solid difference.”