James Van Der Beek Explains Why He Thought He Needed to 'Stop Coffee' After Seeing First Cancer Warning Signs (Exclusive)

James Van Der Beek is opening up about the symptoms he experienced that made a routine colonoscopy all the more crucial — if not life-saving — last August, when he was stunned to learn he had stage 3 colorectal cancer.

"It was just a change in bowel habits," he tells PEOPLE for this week's cover story. "I thought, I probably need to change my diet a little bit. Maybe I need to stop coffee. Maybe I need to not put cream in the coffee. And then I finally took that out of the diet, and it didn't improve, and I thought, all right, I better go get this checked out."

When he went in for the procedure, he said he wasn't worried.

"I felt really, really good as I was coming out of anesthesia that I had finally done it and looked into it," he says. "And as I was coming out of the haze, the gastroenterologist said — in his most pleasant bedside manner — it is cancer."

The actor, 47, says he went into shock.

"I'm very healthy," he says. "I was in amazing cardiovascular shape. I tried to eat healthy as much as I could, as far as I knew at the time. Though I've since learned a lot about what actually eating healthy is."

Van Der Beek says the next stage in his life was his new "full-time job" of having cancer: scheduling appointments, dealing with insurance, cataloging results. But he also learned as he went along about how he could and should change his diet.

"I think the country is waking up to connecting with our food and recognizing just how much it's processed and how far away it's gotten from the way nature intended," he says, of being cognizant of reading labels and looking into just what goes into the food he buys at the store.

"I would encourage everyone to do the same because this is not a fun process," he says of managing colorectal cancer. "There are a lot of assumptions, I think, that we've all grown up with about what's healthy and what's not. And I think it would all do us some good to throw out those assumptions."

Van Der Beek says he's currently "feeling good."

"I'm very cautiously optimistic," he says now. "I'm in a place of healing, my energy levels are great."

His routine these days includes avoiding processed food, gluten and dairy, eating organic vegetables from his garden and exercising regularly. He’s learned to prioritize his mental health. “You need to break down sometimes,” he says. “But I have a lot more self-love and self-acceptance toward myself now... I had no idea how negative my self-talk was before this.”

As for his future plans? He says he’ll be here. “But it’ll be a lighter existence, with a lot more immediate joy,” says Van Der Beek. He appreciates being back at work, including an appearance in The Real Full Monty, a Dec. 9 special on Fox in which celebrities will strip down to to raise money for cancer testing and research.

In some ways, he's glad that he has this chance to change his lifestyle now.

"I really don't feel like this is going to end me," he says. "I really feel like this is going to be the biggest life redirect, and I'm going to make changes that I never would've made otherwise. That I'm going to look back on in a year or five years, 30 years from now and say, 'Thank God that happened.'