Dan Gilroy, Shelley's life partner for decades, tells TMZ ... he found Shelley unresponsive in their home in Texas at 7 AM Thursday. He says Shelley had been under hospice care, and bedridden, for the last few months while dealing with complications from diabetes. He says she died in her sleep.

Shelley had the unforgettable role of Wendy Torrance in Stanley Kubrick's "The Shining" -- she was on the other side of the door when Jack Nicholson smashes through the door with an ax, yelling ... "Here's Johnny!"

Director Robert Altman discovered Shelley in the early 70s in her home state of Texas ... and she went on to star in 7 of his movies, including "Brewster McCloud," "Nashville" and "Thieves Like Us." In 1977, she starred in Altman's "3 Women" ... a role that earned her Best Actress honors at the Cannes Film Festival.

Also in '77, Shelley had a supporting role in Woody Allen's "Annie Hall," and then in 1980's "Popeye" she played Olive Oyl opposite Robin Williams.

She ventured into television in the late 80s, launching and hosting the children's program "Faerie Tale Theatre."

Her final role before retiring from acting came in 2002's "Manna from Heaven."

Then, in 2016, Shelley appeared on an episode of "Dr. Phil" to say she was battling mental illness, and needed help.

Shelley was 75.