The beautiful tomboy tells Vanity Fair she was shocked to learn about Lowe's addiction problem.
"I knew something was happening but I didn't know what" Swank tells Vanity Fair in the August editions set to hit newsstands today.
"When I found out, it was such a shock because I never thought he'd keep something from me. And yet, on another level, it was a confirmation of something I was feeling that was keeping us from being completely solid.
"He's sober now. I don't want to make it seem like that's the sole reason; there were other factors. But that just kind of blew it open. It made me look at things a lot deeper. That's when you realize it's not going to work."
Lowe, 38, and his managers could not be immediately reached for comment yesterday.
Although Lowe's struggle touched off their marriage's downhill slide, Swank said she's proud of her soon-to-be ex's three years of sobriety.
Vanity Fair does not detail the actor's substance of choice.
"It's an enormous obstacle to overcome, and he's doing it. He's living a sober life," said Swank, who married Lowe in 1997. "I know how difficult it is, and I'm really proud of his sobriety."
The actress says she had to stay with Lowe when his substance abuse came to light.
"When I found out, I wanted to be there," she explains. "I knew it was the most important time of his life. That's when he needed me most."
Swank, who won Oscars for "Boys Don't Cry" and "Million Dollar Baby," denied that her skyrocketing career — compared to Lowe's declining job prospects — played any role in their split.
Hollywood watchers will always remember Swank forgetting to thank Lowe, as Oscar cameras focused on him during her "Cry" acceptance speech.
Lowe, already living in the shadow of better-known older brother Rob Lowe, is getting a shot at directing in "Beautiful Ohio," with William Hurt and Rita Wilson.
"Chad is very supportive of my career," Swank says. "He is, I think, genuinely happy for my success ... I think that any frustration for him stemmed from the lack of opportunities in his own career.
Swank has three movies in the can ("The Black Dahlia," "The Reaping" and "Freedom Writers") and is moving out of her 4,000-square-foot Greenwich Village brownstone.
She filed for divorce in March after announcing their separation in January.
"It takes two to make something work or not work. I'm a person with my own faults and troubles," Swank says. "In the end, it just didn't work, but I would never look back on this relationship as failed."
The dark-haired beauty admits she's apprehensive about re-entering the dating arena.
"I thought of it — of that whole world — I can't imagine it! I can't imagine going out right now and trying to find someone else to be with," says Swank, who turns 32 later this month.
Even with her new love-and-life challenges, Swank says she's happy.
"When I told you I'm the happiest I've ever been, it's not because I'm getting a divorce," she says. "It's because I'm living in truth now. My happiness has stemmed from being brutally honest with myself, facing truth every single day, no matter how hard it is."