''The fat thing stopped working,'' says Ricki Lake, sprawling on the bed in the studio apartment in Manhattan's West Village that she shares with her fiance, her dog, two cats, and a Lava lamp. ''Things couldn't get much worse than they already were. I was struggling financially, emotionally. Everything in my life was at a standstill-'' Suddenly her small brown eyes are pulled across the room as her intended, Robert Sussman, leans over the CD player. With an urgency usually associated with fires or severed limbs, Lake whispers, ''Look at his butt!'' With that burst of girlish exuberance, all pathos in the recollection of her low point three years ago is banished. Which is just as well: There's no feeling sorry for the Rubenesque actress/talk-show host now that she's shed 125 pounds, become engaged to a guy who could guest on a ''Sensitive Studs Who Look Like Peter Gallagher'' episode, and turned her syndicated, six-month-old Ricki Lake into the fastest-growing talker on TV, with 3 million viewers a day. Ricki has less sleaze than Geraldo, more sass than Sally Jessy Raphael, but Lake's real model is the Big O. ''I told her I wanted to be the white Oprah,'' she says of her 1988 appearance on Winfrey's show, ''but I never thought I'd have my own talk show at 25.'' Lake, the daughter of a pharmacist and a homemaker, grew up in Hastings-on- Hudson, N.Y., ''wanting to be Annie on Broadway from when I was about 6.'' She spent her youth auditioning, and only occasionally acting; it wasn't until the end of her freshman year at Ithaca College that she met her mentor and savior, John Waters. Casting her in Hairspray (1988) and Cry-Baby (1990), the director turned the zaftig ingenue into a campy cult icon. ''She's one of the only actresses I know who is happy in real life,'' says Waters, whose Serial Mom, due out in April, also features Lake. ''In my movies she's the one person audiences can identify with.'' But Lake attributes her first roles to her rolls. ''I got Hairspray simply because I was fat, so in my head, it was my gimmick,'' she says. ''I didn't have to diet like all of my skinny ingenue friends.'' Gaining another 50 pounds during a 1989-90 stint on China Beach, however, tipped the Hollywood scales against her. ''Then casting directors were saying, 'Yeah, we know Ricki. She's too heavy,''' she says. After one too many movies-of-the-week had slipped through her fingers, forcing her to sell her L.A. house, Lake got physical in 1991. ''Losing weight was the absolute hardest thing. I'm an emotional eater-when I get upset the first thing I think about is food,'' she says. ''Everyone asks, 'How'd you do it?' It's obvious. You've got to not eat for a long period of time and you've got to exercise.'' Lake now stays mentally strong with weekly therapy sessions, works out thrice weekly with a trainer, keeps her cupboards bare, and quit her ''McDonald's for lunch and Wendy's for dinner'' habit. Her career got into shape when supervising producer Stuart Krasnow, who remembered Lake as a lively guest on Late Night With David Letterman, invited her to audition in July 1992 for a new talk show, which she won over 100 others, including Melissa Rivers and model Veronica Webb. ''Around that age, a lot of Hollywood kids turn into jerks, acting like they are better than < everyone else,'' says Waters. ''Ricki doesn't ever think that, and she has a great sense of humor.'' Even her inexperience counts in her favor as she works her take-no-prisoners New York audience. ''I constantly trip, and I'm always flubbing,'' she says, ''so people see I'm a real person.'' Once she pinched a guest who wouldn't shut up on the subject of ''Stay Away From My Man,'' and she thinks nothing of floating off the topic to acknowledge an audience member's nose ring or crazy hairstyle. Lake's spontaneity has paid off in her personal life, too. She met Sussman, 27, an illustrator, at a Halloween party last year and was engaged to him by Thanksgiving. ''We had been talking about marriage from the third day,'' says Sussman, who's Lake's calmer, quieter half. ''On the night before Thanksgiving, I dropped to one knee and officially asked her to marry me. Ricki looked deep into my eyes and yelled 'F -- -, yeah!''' Waters, a minister ordained through a mail-order catalog, will wed the couple in Baltimore this November. Now that the former size 24 is wearing a size 10, Lake also wants to sex up her film career and is searching for a role in a romantic comedy. ''But I want to lose 10 more pounds,'' she says, ''so there's no question that I can do what Marisa Tomei does.'' But she's not fretting too much over the last bit of weight. As she catches a glimpse of Sussman working at his easel, a smile slithers across her face and she shares her proudest piece of news: ''His jeans are too big for me.''