The `Water for Elephants` star - who has daughter Ava, 11, and son Deacon, seven, with ex-husband Ryan Phillippe - finds it hard to combine her career with parenthood, and insists she doesn-t cope as well as people imagine.
She said: "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."
Despite the challenges she is presented with, Reese - who recently married Hollywood agent Jim Toth - can`t help but be driven towards seeking new challenges in her career.
The 35-year-old actress - who won the Best Actress Oscar in 2006 for her role in `Walk the Line` - explained to UK edition of Elle magazine: "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.
"I`m a worker, you know? When I started out, it wasn`t a design that I wanted to accomplish a certain amount of things. But it was, like, finish that movie, it was a comedy, would really like to do a drama now. Do a drama, going to work really hard on this accent. What`s my next challenge? I like to have challenges. That`s fun for me."