The couple split up late last year and Swank filed for divorce in January.
The actress claims Lowe kept his addiction secret, although she doesn't reveal what substance he abused.
She tells Vanity Fair magazine, "I knew something was happening but I didn't know what.
"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.
"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.
"When I found out, I wanted to be there, I knew it was the most important time of his life. That's when he needed me most."
She adds, "It takes two to make something work or not work. I'm a person with my own faults and troubles.
"In the end, it just didn't work, but I would never look back on this relationship as failed. I look at it as 13 and a half years of success."
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