The actress has daughter Ava, 11, and son Deacon, seven, from her previous marriage to Ryan Phillippe, and said that she struggles to cope when her work forces her to be away from them.
She told Elle, 'There's this fallacy that we can have it all. I get it often, 'Oh, you have it all' and I don't. It's hard.
'There are times when I take two years off and I don't work. And there are times I go to work and I feel sick because I can't see my kids for five days.
'And it's real hard to go. My work is important to me. The kids are important to me. My relationship is important to me. But all three of those things can't be functioning at 100 per cent at all times. You'll make yourself crazy.'
The 35-year-old, who married Hollywood agent Jim Toth, added that she has been motivated by various challenges throughout her career.
She said, 'I'm a doer. If there's a task in front of me, I want to do it, get it done, get it done well, wrap it up and move on to the next task.'