Amanda Seyfried left heartbroken by Dominic

Amanda Seyfried was "heartbroken" when she discovered Dominic Cooper was still romancing his former girlfriend after they started dating.

The actress embarked on a romance with her 'Mamma Mia! co-star when they met on the set of the 2008 film and though the British actor had a girlfriend at the time, Amanda mistakenly thought they had split up before they struck up a relationship.

She said: "I was just kind of foolishly thinking that the two of them were done and Dom and I were involved. But we weren't really as involved as I thought. So I got my heart broken pretty hard. And then it took a long time for us to break up again. After that, it's really hard to open myself up to a new person. Really hard."

Explaining how they initially got together, Amanda has revealed they got "carried away" by the romance between their characters in the film.

She told Elle magazine: "[It is] one of the easiest things in the world. You're both open. You're put in a situation where you have to make out with each other. It's easy for things to get carried away."

After splitting from Dominic in May 2010, Amanda embarked on a brief romance with 'True Blood' actor Alexander Skarsgard but it soon fizzled out because she still had feelings for her former lover.

She admitted: "We dated. He's super funny, but I was too involved with Dom."

The 25-year-old blonde beauty has been romantically linked with Ryan Phillippe since last October, but has refused to confirm the speculation.

However, she did say: "Um, yeah. I've been 'seen' with him."