The couple, who married in 2006, had their relationship rocked nearly three years ago when Spelling, 43, discovered that McDermott, 50, had been unfaithful.
The emotional damage done by the affair played out on her reality show, True Tori, and many friends and fans thought the former Beverly Hills, 90210 star would leave her husband. Actually, Spelling says she “never for a moment” wanted to end things.
“Everyone was saying, ‘Divorce him! Why is she staying with him? That is weak,'” she tells PEOPLE exclusively. “But my gut instincts just shut it all out, and I said, ‘I love this man. If there is a way to work it out, I want to try to do that.’ ”
The couple underwent intense marriage counseling and McDermott entered rehab.
“We worked on everything,” says Spelling. “The relationship as we knew it died. We had to bury that and start new.”
RELATED VIDEO: Tori Spelling and Dean McDermott Welcome Son Beau Dean
For the couple, who are already parents to Liam, 10, Stella, 8, Hattie, 5, and Finn, 4 —McDermott is also dad to Jack, 18, from his previous marriage — welcoming baby No. 5 has come to symbolize a fresh start.
“Rebuilding our marriage took time,” she tells PEOPLE. “And now, having a new baby, it makes sense. It’s like a new baby in a new relationship.”
From Coinage: See Where 6 Stars Were Before They Were Famous
McDermott fights back tears talking about it.
“I am so blessed and lucky that she never walked away, because now we have this incredible relationship,” he says. “And I can’t imagine life without Beau.”
- For more on Tori and Dean, pick up the latest issue of PEOPLE, on newsstands everywhere Friday