Cathy Guisewite once thought she was too fat and had a miserable social life. She started a comic strip about a young woman like herself called, appropriately, "Cathy."

Now, two years later, Guisewite is thin, and has a boyfriend, so she has to think up ways to make hereself feel sad so she can write with the kind of tragi-comic pathos that has made "cathy" so successful.

"Cathy" is the infinitely quotable, clippable comic strip about a young woman named Cathy.

When the strip debuted, enter Cathy, caught between her rabidly liberated gifrfriend Andrea, pushing feminism, and her boyfriend Irving whose shirts she ironed. Since then, she has dumped Irving, but still she pines away for a relationship, freezes in the doorway of a party and bemoans excess pounds.

Two years ago, Guisewite was a successful 26-year-old advertising executive. To boost her often drooping spirits she drew little cartoons of her predicaments and sent them home to Mom and Dad. Her parents were so amused they urged her to submit them to a comic strip syndicate. Once Guisewite did so -- her mother threatened to do it -- she had a contract within a week.

Today, 200 papers subscribe to the strip, a book of the strips ("The Cathy Chronicles") has just been released, and Guisewite is a full-time comic strip winters who makes $50,000-plus a year and winters in California. In fact, she is one of very few widely syndicated female cartoonists. (Dale Messick, creator of Brenda Starr, is another.)

"We'd been looking for a strip dealing with women's issues," said Lee Salem, managing editor of the Universal Press Syndicate, which also publishes "Doonesbury." "'Cathy' was the first we saw with some feeling, some soul."

Salem says the cartoon is the first of its kind. And since its debut, it has been a quick hit, according to the syndicate, particularly with the female crowd aged 18 to 30. The mail indicates, however, that young single males as well as grandmothers are reading it.

"When it began we got a lot of comments from feminists saying it was a put-down of women," said Salem. "But that negative response has dropped off. I think they're getting to know her better."

One devoted reader wrote to Guisewite about Cathy, "She's not pretty, always knows she should lose weight and watch her diet, but is terrific at making excuses... (she) does not have it all together like her libber friend Andrea, but she's trying! She's disappointed, lonely, and is hung up on a typical male. In short, she's one of us."

Indeed she is. "Cathy just came about very naturally from my frustrations," says Guisewite. "I don't think I could make a comic strip about something else."

Guisewite observes herelsef in action. "I almos left an argument I was having with a guy so I could write down what the argument was about," she says.

Guisewite hereself is always struggling to break a two-pack-a-day cigarette habit."I usually have Cathy try to quit every January." (This year Cathy of the comic strip only made it through the first few days of January.) Guisewite's talks with her mother sometimes end up in the strip. "Some of what Cathy's mother says in the strip are direct quotes from my mother," Guisewite says.

When Guisewite recently mustered enought courage to have her long brown hair trimmed a few inches to just below her shoulders, she sent Cathy in the strip off to the beauty salon for a permanent and a new look -- which fizzled. "I just extended my own fears to Cathy."

In another strip, Cathy slides cautionsly into a party and stands at the doorway, looking around, in an attempt to determine if everyone else in the room is skinnier than she is.

"I think I go through all of that," sasy Guisewite. "I scrutinize people when I walk into a party. I worry about what to wear, about fitting in. I feel all the same basic insecurities.

Still, Guisewite says the strip does not chronicle the daily affairs of her personal life blow by blow. Infact, at the beginning, she was too embarrassed to use her own first name for the title of the strip. But the syndicate convinced her it was the best choice. "I went through a book of names," she says, "and couldn't find any that I liked better for the strip than Cathy."

When Guisewite is not in California, she lives outside Detroit in Southfield, Mich., in a three-bedroom condominium, one room of which she has turned into a studio with desk, drawing table, color TV, and stereo -- "the essentials."

She writes and draws the strip on different days. For writing, she closets hereself in her apartment for a few days and puts on unrequited love songs. Preferably Janis Ian and Jim Croche. "For some reason I have to get depressed to be funny," she says.

On another day, Guisewite Will spend about two hours drawing one strip. It used to take all day. "I'll always be grateful to the syndicate," says Guisewite, whose previous art experience was drawing little greeting cards and books for family and friends. "They were always confident I would learn how to draw."

when she is not feeling funny, she procrastinates. "I go to eat things or I do the laundry." she says. "I've finally rationalized that procrastination is essetail to the writing process." So had Ernest Hemingway who, when asked how to write a novel, said, first clean out the refrigerator.

Before the comic strip Guisewite was a vice president of the W. B. Doner advertising firm. "I worked real hard," she says. "I really throw myself into what I do. That's probably my greatest asset.My work becomes my entertainment. That's the way I felt about the agency and that's the way I feel about that strip. Part of my personal problem is that my main lover is my work."

Guisewite describes herself as a cross between Cathy and Andrea, the two female characters she has created. She is vulnerable like the first. The second is her conscience.

"When I read about the liberated woman I think I will never be that way." Guisewite says. "She too powerful. She has every detail of her life worked out. The ideal seems so out of reach, yet it seems like something worth going after. A lot of what Cathy goes through is this searching for the qualities of the new woman."

If Cathy of the strip ever finds them, chances are it will be because Cathy Guisewite has found them first. Comic-strip Cathy may become more assertive or find herself in a new situation but only if Cathy Guisewite has already done so. "I don't think if I got married that Cahty would remain single," Guisewite says. "She would probably have to get married."

If life becomes blissful for Guisewite, what will happen to Cathy? "I wonder about that somethimes. Well, I don't think anyone is completely happy. There are always insecurities in life that will make material for the strip." CAPTION: Illustrations 1 through 4, No Caption, Copyright (c) 1978, Universal Press Syndicate; Picture, Cathy Guisewite