Denise Richards' considers the death of her mother to be her "biggest struggle."

The 41-year-old actress, who is mother to two daughters with Charlie Sheen, was considerably impacted when her mom passed.

The pair had a tight-knit relationship and Denise couldn't imagine life without her.

"Losing my mom four years ago to renal cell cancer has been my biggest struggle," she told the US edition of OK! magazine. "She was 53 and we were incredibly close. Not a day goes by that I don't think about her. My girls have been my pillars of strength."

Denise turned to food initially to deal with her grief.

"When my mom passed away, I gained 15 pounds," she shared. "This was the first time I understood what emotional weight was, it was more difficult to lose this than my pregnancy weight. I made a strong commitment and started doing Pilates six times a week and lost the weight."