Janet looked in the mirror, blinked her large blue eyes at herself and smiled. "Oh you beautiful creature," she seemed to be saying as she arranged each strand of her blond mane into a windblown halo.

She weighed 250 pounds, but she saw a Farrah Fawcett-Majors twin smiling back at her. . .

Susie, a once-plump teen-ager, glared critically at her size-5 silhouette in her mirror. A hint of bulge, invisible to all but her own merciless eye, drove her into a paroxism of self-disgust. She saw only the chubby body she'd left behing years ago. . .

These are extreme examples, but weight-managment therapists, doctors and researchers are finding that misperceptions and distortions of a person's self-image are important factors in weight loss and maintenance.

Between the Janets of the world, who may never even try to lose weight because they don't see themselves as fat, and the Susies, who can never see themselves as anthing but fat, is almost everyone else who has a weight problem.

One of the most insidious effects of this common psychological blindness occurs among people who have lost 20 or 50 or even 150 pounds, agonizing pound by pound, yet do not see themselves slimmer.

Sandy Daston, weight connselor at the Georgetown University Medical School Behavior Modification Weight Control clinic, calls this problem "phantom body size," and finds it the most common type of self-image distortion. "It is," she told a group of participants in the program, "much like a person who has lost a limb, but still has perceptions of sensation in the arms or leg that is no longer there."

Even though the formerly fat person knows - intellectually - he or she is now really thin, the psychological distortion can transform the bedroom mirror into that of a nightmarish fun-house.

Typically, a no-longer-fat woman will continue to wear the same "baggy, hangy clothes" under which she hid when she was grossly overweight. One man who lost 100 pounds still never sits down in a bus or train unless two seats are empty, and then he is shocked when someone sits down beside him - and fits.

The trouble is, says Daston, if the formerly fat person cannot grasp the feeling of being thin, he or she is most likely to regain all the lost weight in a perverse "If I still feel fat, I might as well be fat" chuck-it-all despair.

For many people, especially those who become fat in adulthood, adjusting to being thin may be mainly a matter of time, a sort of "fat lag," like jet lag. But for those whose fat self-image was forged in childhood or adolescence, readjustment may not be so easy.

A lot of the problem, says Daston, is all tied up with a person's reasons for being fat in the first place.

It is not always true, as conventional wisdom dictates, that inside every fat person is a thin one screaming to get out. Often it is a scared person who is afraid to come out.

Our society looks upon fat as bad (read "ugly," "weak-willed," and a lot of other negatives). Daston, of course, advocates weight loss, but she feels this attitude is unfortunate "in the sense that most people are out of control about something in their lives. It's just when it's food, it shows a little bit more."

The constant barrage of "this-is-beautiful-and-good" propaganda has to affect the self-image of people with weight problems.

Then, there is always a let-down after losing a lot of weight. During the reducing process, there is positive reinforcement from others that comes naturally. People say, "On, you've lost so much weight,". . ."How wonderful you look". . ."How did you ever do it". . .

People after all, are going to get used to the new look.They're just not going to say, "Oh, how wonderful! You're just as thin today as you were yesterday."

But one craves that reassurance that the change is real.

Other obstacles to seeing oneself clearly:

The prevalent misconceptions among the fat-afflicted that thin people have no problems, that thin people have always been thin and that slimness will be a magical cure-all.

The occasionally negative attitudes of other people. The insecure thin person - there are lots of them - may have been comfortable in a relationship with a fat, jolly person to whom he or she felt superior, but may feel much less comfortable with a newly thin competitor . Of course, it's the thin person's problem, but the dieter may not realize that.

Weight counselor Daston offers clients these suggestions for adjusting mentally to a smaller physical size:

Exercise. This is especially useful for several reasons. It places the Body in space, gives a sense of accomplishment and helps the body's hunger gauge work more accurately, helping to prevent the mistaking of emotions such as anger, frustration or loneliness for physical hunger.

Periodic self-examination of one's scantily clad or nude body in a full-length mirror, perhaps comparing the new image with photographs taken before and during the weight-loss period.

3. Buying at least one piece of clothing of the right size every 20 pounds or so. One program participant said she used a piece of clothing that was too tight by trying it on periodically as she lost weight. When it had gotten too big, she knew she was smaller.

Use a tape measure, to show yourself that you've shrunk by real inches.

Beauty is in the eye of the beholder, sure. But you want to be seeing straight when you behold yourself.