Katherine Heigl has admitted that she cannot give up working because motherhood "cannot fulfill everything" in her.

The Knocked Up actress adopted her daughter Naleigh from Korea two years ago and says that she loves her baby girl.

But the 33-year-old reveals that balancing being a parent with her career is a lot tougher than she imagined it would be.

"I have always known I wanted to be a mother," she tells iVillage in a new blog post. "There was something about the weight of a child in my arms their soft cooing, their sweet milky smell, their giggles and even their tears."

"I have also always known I wanted to be an actress. From the moment I walked onto my first set when I was 11-years-old, I knew I had found my passion."

But Katherine says that she realised "that having it all would not look and feel as I imagined."

"I knew, of course, as I prepared to welcome my daughter into my life that it would be a bit of a juggling act but I had no doubt that I would do it," she shared.

"I went into it full throttle, ready to buckle down and make it all work seamlessly as I always imagined I could. The thing is I couldn't."

"No matter how great my intentions, how lofty my goals, how passionate my commitment, I was failing. I was failing my work, I was failing my daughter, I was failing my husband," she explains. "I couldn't help but wonder what I had gotten myself into and if the choice I made to be a working mother was the most selfish decision of my life."

Katherine questioned what was wrong with her and how she could leave her child to go back to work and miss the important moments during the day but came to an honest conclusion.

"The simple truth is that I love my daughter passionately and as most mothers do, think she is the smartest, funniest, prettiest child in the whole world but she cannot fulfill everything in me," Katherine said.