The 'Fashion Star' mentor, 32, who fell pregnant four months after giving birth to baby Maxwell, said, "Motherhood is the best thing I've ever experienced - and the most challenging."
Balancing her career with her personal life is one of Simpson's struggles, reported Us magazine.
"On some of the days of Fashion Star, I'm awake before [my daughter Maxwell] is awake, and I'm not home until she's already back to sleep," she said.
"That's only happened about four times, but it makes the day awful. I definitely need to see my baby.
"I get really sad if I don't get to see her. And then when Maxwell sees me, she just stares at me and touches my face. I can tell she missed me. That breaks my heart."
But at the end of the day, Simpson and her fiance Eric Johnson "feel like we've done such a good job" parenting thus far.