Her trademark toothsome smile has never dimmed, but Mary Tyler Moore, 45, has seen more than her share of troubles over the last few years. In 1978 her 21-year-old sister, Elizabeth, died of a drug overdose. Two years later her only child, Richard Meeker, 24, died of a self-inflicted gunshot wound in an apparent accident. And in 1981 her 18-year marriage to Grant Tinker—cofounder of MTM Productions and now chairman of NBC—ended in a painful divorce.

These days, Mary—who was set to wed Dr. Robert Levine at Manhattan's Pierre Hotel on Thanksgiving Eve—is said to be rapturous. Her 30-year-old partner, a Loyola-educated cardiologist at New York's Mt. Sinai Hospital, seems to have all but cured the heartsick actress. "You'd never know this is her third wedding," says one friend. "All she talks about is her dress and how excited she is."

"Mary's filled with joy because she and Robert are friends as well as loving each other," explains MTM's longtime TV co-star Valerie ("Rhoda") Harper, who adds that she is "thrilled" to be a bridesmaid. "He makes her feel cherished. Both families [Levine's is Jewish, Mary's Roman Catholic] are happy, too."

Some of Moore's other close chums have been less blissful about the prospective marriage. One intimate complains that Levine is "a big bore who's never been around or done anything interesting. All he talks about is medicine. Besides, he's unattractive." Adds another confidante: "I saw her entwined with that shlub on Columbus Avenue. What does she see in him?"

As one observer interprets the affair, Mary may be trying to even the score with Tinker, 57, who for some time has been dating a former secretary, Melanie Burke, in her mid-20s. Mary was "devastated by the breakup," claims the friend, suggesting that Moore began seeing the younger Levine "to get back at Grant."

However untraditional her choice of grooms, Moore was feted last month at a bridal shower at the Park Avenue apartment of producer Dasha (Whose Life Is It Anyway?) Epstein. Eight guests, including actresses Pippa Scott and Hope Lange, brought alarm clocks, stethoscopes and lingerie designed to keep the doctor in the house.

When it comes to selecting a younger man for a mate, Moore is hardly alone. Current May-September couples include Linda Ronstadt, 37, and comedian Jim Carey, 21; Olivia Newton-John, 35, and actor-dancer Matt Lattanzi, 25; Carly Simon, 38, and actor Al Corley, 27. It's a trend that could be catching. New York psychologist Penelope Russianoff, who played Jill Clayburgh's therapist in An Unmarried Woman, says, "Young men are taken by the honesty of a woman who has the guts to go against cultural clichés."

Psychotherapists use adjectives like "successful," "curious," "self-confident" and "unconventional" to describe the kind of older woman who selects a youthful lover. But society often sees things differently. "People say, 'She's robbing the cradle, and he's marrying his mother.' That's rotten," declares Russianoff. "I've had such couples in counseling and I've never felt that was the case."

Moore, for one, doesn't seem unduly concerned about the whispers. Although she and Levine claimed to be mere pals before their October engagement announcement, she has taken few pains to keep her romance under wraps. In July the two rented a sprawling house in Long Island's old-money Quiogue, near Westhampton, where they lazed by the pool and dined in public daily. And during Rosh Hashanah, Moore and Levine (whose family is said to be "quiet and very religious") attended synagogue services in Manhattan.

Can Mary and her heartthrob succeed in bridging the generation gap? One friend worries that Mary's trip to the altar may be hasty: The relationship, she says, "would make a better affair than a marriage." But Russianoff thinks they might just make it, after all. "Older women and younger men have a great deal to give one another," she says. "Besides, why is this different from an older man marrying a young, skinny cutie? People would say, 'Wow—more power to him!' "