Justin Theroux is sharing a heartbreaking story about a former relationship.

On Wednesday, the 46-year-old actor attended the Los Angeles premiere of his upcoming movie, The Spy Who Dumped Me, where he dished about catching an ex-girlfriend in bed with another man. When asked if he's had any traumatic breakup experiences, Theroux told E! News' Zuri Hall that he's had "heartbreaks."

"I walked in on a girlfriend, after coming back to her, like we were separated in the summer, I was working and she was crying on the phone to me all the time going like, 'It's our last summer together,'" Theroux shared, explaining that he was about to go off to college at the time. "And I came back and she was schtupping a guy who was like, in her bed."

Justin Theroux, The Spy Who Dumped Me, Premiere

Matt Baron/Shutterstock

"It was a terrifying experience," Theroux continued. "I went to surprise her! A romantic thing of like drive all the way up and see her."

Theroux added that he's learned, "It sucks whether you're breaking up with someone who really likes you or if they're breaking up with someone and you really love them, it's the worst. There's no good side of that coin to be on."

Watch the video above to see Theroux tell the heartbreaking story, reveal what it was like working on the movie and give breakup advice!

The Spy Who Dumped Me hits theaters on Aug. 3.

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