The actress began seeing Cooper after they met on the set of Mamma Mia! and admitted that she felt "foolish" when she found out he was still seeing his ex.
She explained to Elle: "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."
The 25-year-old explained that they both got "carried away" while they were starring in the musical.
She added: "[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."
Seyfried also addressed rumours that she is dating Ryan Phillippe, cryptically saying: "I've been 'seen' with him."