Rose McGowan is trying to heal the rift with cancer sufferer Shannen Doherty after replacing her on TV series Charmed 15 years ago.
The actress posted a cartoon drawing of herself and Shannen, who is fighting breast cancer, and blamed Hollywood for making them see themselves as rivals.
In a message addressed to Shannen, 45, she wrote: "We have a funny history. As young women we were pitted against each other for society's pleasure. The rules of Hollywood engagement brainwashed into us were truly vile."
The 43-year-old said she regretted the two had not been friends.
Shannen left the series about sister witches after the third season in 2001, amidst reports of a fall-out with co-star Alyssa Milano.
She explained: "We were cast in this weird fake reality show where we were supposed to be enemies. I resented it greatly. I regret not being awake enough to articulate this to you at the time.
"Fear was drilled into me from day one in this town & you were the barometer by which my behaviour was judged. I was under a microscope, one false move and I'd be branded 'difficult' just like you. I think we all know what happens when girls here get out of line.
"I wilfully remained enigmatic & un-engaged. The Bad Girl Shaming you received at the hands of the media was merciless."
She added: "I had to prove I wasn't as bad as you or I'd get the axe, too. But we were never the bad ones, it was them."
Rose was drafted in as a replacement lead actress, playing Paige after Shannen departed the show.
Shannen is now nearing the end of radiation treatment and chemotherapy, having undergone a single mastectomy.