"I knew something was happening, but I didn't know what," Swank said. The two-time Oscar-winning star of "Million Dollar Baby" and "Boy Don't Cry" did not name the substance, but said Lowe is sober now. When she became aware of his habit, she said she wanted to be there to help.
"That's when he needed me most," she said.
Substance abuse was not the only reason for their breakup, but "that's when you realize it's not going to work," she said.
Swank, 31, said jealousy over her success was not a factor in the split. She said Lowe has always been supportive of her career.
The couple announced their separation in January, then attempted to reconcile, but ended up divorcing in May. They had no children.
"In the end, it just didn't work, but I would never look back on this relationship as failed," said Swank. "I look at it as 13 1/2 years of success."
2014: The Year in Fashion [PHOTOS]
Naya Rivera supports her brother in Raiders cap