The Irish singer, who split with the PRIDE + PREJUDICE actress last year (05), claims her fame and fortune made him feel inferior and insecure.
He says, "You can feel bit second rate and that's what started to happen.
"There is a big pressure when you go out with someone such as Keira.
"The man is meant to be the alpha in the relationship on the money and the power front and clearly I was not.
"You feel like you have to be dominant in other areas and that causes problems.
"Keira could see what I was going through. She was the one who finally said, 'Ok, that's it, I've had enough.'"
Dornan remains friends with Knightly - who is now dating her Pride + Prejudice co-star Rupert Friend.
He adds, "She's the most beautifully grounded person I have ever met."