03/08/2011 AT 11:00 AM EST
Amanda Seyfried may be "very much a couple"
with Ryan Phillippe
these days, but moving on from her relationship with ex Dominic Cooper wasn't an easy thing to do.
"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.
Seyfried says staying together for so long made it hard when they finally split for good
almost a year ago.
Calling falling in love on set "one of the easiest things in the world," the Red Riding Hood
star explains, "you're put in a situation where you have to make out with each other. It's easy for things to get carried away."
But Cooper wasn't the only man Seyfried met on the set of Mamma Mia!
She also got cozy with the son of costar Stellan Skarsgard, True Blood
's Alexander Skarsgard, 34.
"We dated," she says of the younger Skarsgard. "He's super funny, but I was too involved with Dom."
Now with current beau Phillippe, 36, Seyfried is much less forthcoming when it comes to talking about romance. "Um, yeah," she says. "I've been 'seen' with him."
The duo first sparked
relationship buzz when they were spotted leaving Kate Hudson
's Halloween party together in October.
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