updated 03/08/2011 AT 10:05 AM ET
•originally published 03/08/2011 AT 11:00 AM ET
“I got my heart broken pretty hard,” Seyfried, 25, says in the April issue of Elle magazine. “After that, it’s really hard to open myself up to a new person. Really hard.”
The actress, who dated Mamma Mia! costar Cooper for more than three years, says she was initially hurt because he was still involved with someone else.
“I was just kind of foolishly thinking that the two of them were done and Dom and I were involved,” says Seyfried, who split temporarily with Cooper, 32, before getting back together.