07/11/2012 at 05:00 PM EDT
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."