One morning in her 40th year, actress Marsha Mason awoke in the Bel Air estate she then shared with husband Neil Simon, dragged herself out of bed with unaccustomed lethargy and stared into a mirror at the prom-queen face with the upturned nose. "Who are you?" she asked with the same dramatic intensity she had brought to four Oscar-nominated film performances.

Mason's mirror scene was the beginning, she says, of a "major mid-life crisis" that lasted more than a year. "I thought I wasn't attractive or talented anymore. I cried easily and was depressed and removed. I became emotionally insecure about what the second half of my life would bring." Indeed, around a year later, her 10-year marriage to Simon fell apart, leaving her, she says, "angry, scared, frightened and lonely. I had thought the marriage would last forever."

But after her initial despair, Mason, now 43, has rallied to pursue a new career as a stage director. She made her debut with a production of Heaven Can Wait starring Robert Hays at the Burt Reynolds Theater in Florida and then decided to brave New York. Last week her Juno's Swans, starring Mary Kay Place and Betty Buckley, opened off-Broadway. The gentle comedy is the story of two sisters, one a Midwesterner, the other a sloppy, neurotic New York actress, who meet again after a long separation and share a lover. "Directing is much harder than acting," says Mason. "I blew it a few times, and sure, I told the cast. I said, 'Guys, I'm up a tree here.' "

The rookie director has always taken risks in life. A printer's daughter from St. Louis, she was educated in parochial schools and confounded the nuns by her passion for speech, debating and student theater. After studying drama at nearby Webster College, she headed for New York. Love of Life, road shows, off Broadway and, finally, classical repertory acting with San Francisco's American Conservatory Theater led to her first movie break: Paul Mazursky's Blume in Love (1973). Along the way came a five-year marriage to a fellow actor, Gary Campbell, which ended in 1969.

Then one day in 1973 Marsha auditioned for Neil Simon's Broadway play The Good Doctor. "You're the girl from Love in Bloom," said Simon, who isn't in the habit of bollixing up words. Something was in the air. As devotees of Simon's autobiographical Broadway hit Chapter Two and the movie version well know, 22 days after that meeting, Neil and Marsha married.

The new Mrs. Simon promptly abandoned acting for two years to spend time with widower Neil and his daughters, Nancy, then 10, and Ellen, 15. "Early on in life I didn't think I'd make a good mother. I thought I'd visit my craziness on a child. With Ellen and Nancy, I learned that wasn't true."

As the girls matured, Marsha resumed her film work and starred in Simon's The Goodbye Girl, Only When I Laugh and Chapter Two, which won Mason three of her Oscar nominations. (The other was for her first starring role in Cinderella Liberty in 1974.) The next Simon-Mason team effort, Max Dugan Returns, was not so well received. About the time of its release, the Simons announced a separation. "It was very sad for both of us," says Mason. "I'm sure my mid-life crisis aggravated the problems, but it was all very, very complicated."

After the turmoil and anger, says Marsha, "Neil and I tried really hard to accomplish a friendship." He recently saw her play; she saw his (Biloxi Blues). Marsha also remains in close touch with Neil's daughters. Nancy, now 22 and an aspiring director, was Marsha's assistant on Juno's Swans.

But there is more than simply Simons in Marsha's life. An earnest, introspective woman, she now envisions the future as a time to combine acting, directing, producing and even short-story and screenplay writing. Thus, she's on the lookout for new plays to direct and also plans to co-produce and co-star with Jane Alexander in The Washington Project, a film written by Barbara Howar about two women confronting the D.C. power structure.

Then there's the matter of reestablishing the unmarried life. "I date. I have men friends," Mason says with a self-conscious little smile. Meanwhile she's settling into single nests on both coasts. In Los Angeles, where Marsha spends most of her time, she rents a "sweet, two-bedroom cottage" in Brentwood. In Manhattan she is living in a hotel while awaiting the completion of renovations on the first piece of property she has owned alone: a two-bedroom apartment overlooking Central Park. Stopping in to view the progress one afternoon, she steps gingerly over the piles of sawdust and rubble. "It's terrifying and exciting," Marsha says, expressing a sentiment that encompasses not only the dangling wires and exposed pipes but also the delicate Masonry ahead in Chapter Three.