Her Shocking Secret

updated 10/05/2009 AT 01:00 AM EDT

originally published 10/05/2009 AT 01:00 AM EDT

One of the main things I want to say," says Mackenzie Phillips, "is don't hate my father. Drugs will make you do stuff that you would never, ever do." What her father, John Phillips—founder of the famed '60s pop group the Mamas and the Papas—did, horrifyingly and incomprehensibly, was have a sexual relationship with his oldest daughter. Burdened by that dark history for 30 years, Mackenzie Phillips, 49, decided to share it, along with other tales of her days as Hollywood's preeminent wild child, in her new memoir, High on Arrival. The decision to come forward wasn't easy. Her brother Jeffrey, 51, and half siblings Chynna, 41, Tamerlane, 38, and Bijou, 29, "aren't happy," she tells PEOPLE's Joey Bartolomeo. "But I had to tell the truth." Arrested for heroin possession last August, she says she is now drug-free, sharing her San Fernando Valley home with her musician son Shane, 22, and set to appear as a mentor on the new season of VH1's Celebrity Rehab with Dr. Drew, premiering in January. The actress hopes her story will help other incest victims. "And maybe," she says, "it will ultimately bring this family closer."

My mother, Susan Adams, kept an ordinary, clean house. She was sweet and warm, and she knew how to make life fun for my brother Jeffrey and me even if there wasn't much money. There was laughing, singing, and playing dress-up.

Weekends, we entered another world. A limo would transport us to Dad's mansion in Bel Air or his mansion in the Malibu Colony, where our childhood veered down a psychopharma rabbit hole. The house was a bizarre mix of excess and oblivion, luxury and incompetence. I swam naked at midnight and scrounged for dinner. I had no idea what I might hear or see.

After John and second wife Michelle (who was also in the Mamas and the Papas and is Chynna's mother) broke up, he began dating and then wed actress Genevieve Waite (mom to Tamerlane and Bijou.)

My father loved me, but he didn't see himself as my protector. He and Genevieve had no qualms about dipping into a bowl filled with white powder and inhaling it in front of me and my brother one night. After they left, Jeffrey, who was age 12, said, "Let's try it." It made me feel grown-up.

When she was almost 13, Mackenzie (whose first name is Laura) ran away from her mother's house to live full-time with her dad in Bel Air.

Dad gave me my own wing of the mansion. I was supposed to go to school, but sometimes I had to hitchhike because nobody was awake or willing to drive me. I was allowed to go to school on acid if I wanted, and Dad signed papers on which I'd write excuses. I never dared write "Please excuse Laura from school as everyone is just too f----d up."

Eager to act, she began going to auditions. At 15, she was cast in One Day at a Time, a soon-to-be-a-hit sitcom costarring Valerie Bertinelli.

Val's innocence made me feel sophisticated. But I wasn't all confidence. What teenager wouldn't feel ugly next to Val? My bad skin was a particular source of angst. No matter, Val and I formed a special bond. We had our lunch hour off, when we'd relax at my house, each having a glass of wine poolside.

Mackenzie's drug use escalated, and a week after her 18th birthday, high on quaaludes, she was arrested following a night of clubbing.

When it became clear that I was bad press, not to mention a bad influence, Valerie started backing off. I can't blame her. It makes sense to detach from someone who's having so many crazy problems. But it hurt me. I always loved her. I still do.

Not long after her arrest, Mackenzie attended a party with her dad and Mick Jagger. The trio retreated to Jagger's apartment to make sandwiches; at one point John left to borrow mayo from a neighbor.

The minute the door shut behind him, Mick locked it. He said, "I've been waiting for this since you were ten." We went into his and Jerry Hall's bedroom and had sex in their bed. Maybe most people think their parents' friends are old and gross. But this was Mick Jagger! He was hot.

In '79 Mackenzie met Jeff Sessler, who worked for Keith Richards; after a drug-fueled fling they decided to wed.

On the eve of my wedding, my father showed up, determined to stop it. I had tons of pills and Dad had tons of everything too. Eventually I passed out on Dad's bed.

My father was not a man with boundaries. He was full of love, and he was sick with drugs. I woke up that night from a blackout to find myself having sex with my own father.

Had this happened before? I didn't know. All I can say is it was the first time I was aware of it. For a moment I was in my body, in that horrible truth, and then I slid back into a blackout.

Her wedding plans fizzled (but they eventually did marry); a deeply shaken Mackenzie told her mother what her father had done.

She said, "Are you sure?" She couldn't process this. My dad's sister Rosie believed me and was furious. I wondered if I should press charges. But Rosie helped me realize that it would not only ruin my father's prospects of ever finding success again, it would also taint me. And my family. It would destroy us.

Months later: I said, "Dad, we have to talk about how you raped me."

He said, "Raped you? You mean when we made love?" That statement captures my father. If sex happened between a father and daughter, and nobody protested, there was no problem.

Fired from One Day at Time in '80 because of her drug use, she went to rehab—with her father—and later toured with him in the New Mamas and the Papas. On tour she fell in love with guitarist Mick Barakan. By this time, though she told no one, her sexual relationship with her father had become consensual.

I was a fragment of a person, and my secret isolated me.

One night Dad said, "We could just run away to a country where no one would look down on us. There are countries where this is an accepted practice. Maybe Fiji." He was completely delusional. No, I thought, we're going to hell for this.

The intermittent relationship (it was during an off period that she had son Shane with Barakan) ended in the early '90s, only after Mackenzie learned she was pregnant and, worried John could be the father, had an abortion. She went to rehab again in '92, and in '01, as her dad lay dying of heart failure, she tried to talk to him.

I felt timid. My dad could cut you with a sentence. I steeled myself and said, "We've been through a lot. You know what I'm talking about ... I would not be the woman I am had I not been your daughter. So I want you to know that I forgive you, and I love you very, very much."

Dad looked up at me. As I expected, he didn't say anything; he just sighed and put his head on my shoulder.

Clean for close to a decade, Mackenzie became addicted to painkillers after liposuction and breast implants in '01. Eventually she moved on to heroin, and following her '08 arrest, she spent three months in rehab.

I don't feel like an addict. I don't fight an urge to use drugs. My monster is a quiet sleeper. But I don't ever forget who I am.

As for what will become of me, I will always be Shane's parent, and that is all I really want. To be a good mother and sister and friend. To work in my chosen profession. To be humble and happy in the precious life I nearly lost so many times. I am amazed to be alive, and I am wildly in love with this great life.

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