"Stress isn't something I really allow into my life. It creeps in but that's a huge thing, changing my diet, changing my want to make everyday the best that I can," Applegate, 38, told PEOPLE over the weekend at the Entertainment Industry Foundation's annual Revlon Run/Walk in Los Angeles.
The actress and her support system – which includes her mother and fellow breast-cancer survivor, Nancy Priddy, friends and 20 crew members from Samantha Who? – were part of the group of 40,000 people who turned out at the event to help raise money for women's cancer research.
While Applegate, who under went a double mastectomy, admits she's not immune to negativity, she tries to talk herself out of it. "I fall short. I get into self-pity and all of those things you do ... but at the end of the day, this is the day that I'm living and I need to rejoice in it," she says.
At the moment, Applegate is rejoicing in her role as the future wife of musician Martyn Lenoble, whom she called her "best friend."
"This chapter of my life is bigger, better, more exciting. I'm looking forward to everything my life has to offer," she says. "I feel incredibly grateful."
Also, she's not about to let the stress of wedding planning get to her.
"We don't even know when [the wedding will be]," she says. "We're just living right now."