The actress/TV host gets very candid in her new memoir, 'Bits and Pieces,' about how she was doing drugs from an early age -- only to get clean in the '70s ... and then dive back into it again in the '80s as she was starting to hit the Hollywood scene.

Whoopi says coke was her biggest vice ... as she explains it was quite literally everywhere, and being dispersed with such ease at parties and get-togethers between NY and L.A.

She writes, "I was invited to parties where I was greeted at the door with a bowl of Quaaludes from which I could pick what I wanted. Lines of cocaine were laid across tables and bathroom counters for the taking."

Whoopi goes on to say that she was knee-deep in cocaine use, and that everything was under control for about a year -- but at one point, she says it started to "kick my ass" ... causing her to have hallucinations and confine herself to her room for long periods of time.

In fact, she describes one scary episode where she was so high she thought there was a monster under her bed ... and she stayed there for a full day, peeing the bed too.

The moment she got a wake-up call came when a hotel maid found her doing coke in the closet ... and Whoopi saw herself in the mirror -- with white powder all over her face -- she realized she had a major problem and had to get clean (again). So, that's what she did.