"This is the first letter I have ever written to an advice columnist," writes a mother with an only child:

"I always thought the people who wrote such letters were the same ones who talked to themselves on street corners. Maybe so. I feel less stable each day.

"My son is 3. When he was born, I was 31 and my husband 40. We wanted a baby badly and consequently have made him the center of our whole world -- far too much for his own good. Nevertheless, until recently he seemed only slightly spoiled. He went through the "Terrible Twos," but didn't seem much worse than other kids. As he approached 3, there was a definite improvement.

"He is bright and very verbal, and for awhile was fairly cheerful and friendly. In September (just before his third birthday) he started nursery school three mornings a week. He liked it and loved his teacher, but cried when I left. Now he tells me good-bye on the sidewalk and there are no more tearful partings.

"Now we come to the problem: About three weeks ago, he turned into a monster! He is making our lives miserable (and his own, of course). I have tried various methods of handling the situation, but am just not coping.

"He wakes up whining and throwing tantrums about some obscure thing. Usually he has wet the bed, is mad about that and mad about having to change clothes. All day long I hear, 'I don't like you! I'm going to throw you in the garbage! You yucky! You bad! You pee-pee!'

"yhe whines, picks his nose, spits, drools, puts things in his mouth, refuses to share, snatches toys away from others, hits at the slightest provocation, talks baby talk, runs away from me in stores or outside and won't return when called. Our evening meal is especially miserable. He eats very little now (refuses anything remotely resembling a vegetable) and spends his time making messes and interupting our conversation.

"I know this has got to stop, but our attempts at discipline have been dismal failures. When he is sent to his room, he either refuses or pops right out again acting even worse. Sitting in the corner is more of the same. We had a latch on his door when he was younger, but took it off recently at his request. I neglected to mention that he seems obsessed with 'monsters' lately and works himself up into near-hysteria when closed up in a room alone.

"Taking away privileges doesn't really work. He does love TV cartoons, but I'm trying to wean him away from them anyway. I had thought that Tom and Jerry' and 'Bugs Bunny' were relatively innocent, but now I wonder if some of his violent behavior isn't learned there.

"One last thing: I made the mistake of sitting by his bed in the evenings while he went to sleep (when he first started having dreams and fears of monsters). He is usually tired and goes to sleep quickly. But now it has become a habit and it is no longer possible to leave his room until he is asleep.

"I hope you have some suggestions. A few years ago, I would have known exactly what a mother should do. He won't go to his room? Then you just give him a good spanking. Now having given some of those and lost my temper to an embarrassing degree I know that (1) He only becomes more stubborn, and (2) I feel more frustrated and guilty. Help!"

A. It's as if every baby is born with a present for his parents: a hair shirt -- and it's almost always the mother who wears it.

Perhaps it's the residue of our Freudian age, but today's parents think they are the sole source of any problem their children have. That's a crock.

What we do or don't do definitely affects the child, but not to the exclusion of everything else.

A sudden, severe change in a child isnot only a cause for concern, but a sign that it comes from the child. You can't explain it by saying that he is near the mid-Three stage -- since this is only a mildly cranky time -- or that television is violent or he needs more discipline.

Instead he needs a first-class physical, good records for the doctor and a lot of loving to boost his self-esteem, so he will quit calling you all those rotten names he really is calling himself.

As for the records, they should cover everything that has changed in your child's life since he became so different.

In your letter you said his misbehavior began about three weeks before -- which puts it starting just a few days before Halloween.

All those monsters and gremlins a child sees that night can be terrifying, especially if they got exaggerated in his dreams. We once knew an otherwise dandy Mid-One who got scared everytime we drove over a bridge. With time the fear diminished, but didn't go away until that child said, "Remember that time when our car fell of the bridge? I was so scared." It took a lot of reassurance to make her realize that the memory was a nightmare.

To explore this possibility, talk about your own dreams and ask about his, especially when he first awakes. This helps him sort the confusion in his head. He also needs an open door, a night light, a flashlight by his bed (tape it shut) and that super paperback, "Bedtime for Frances" by Russell Hoban (Harper and Row, $1.95). It takes the fears of a bear quite seriously and then dispels them with quiet wit and reasoning.

Halloween suggests another potential for trouble: candy. If you give your child a piece or two every day from his grimy little hoard, he may be reacting to dye, to chocolate, to a preservative, or even to an overdose of sugar. Allergies can cause the behavior you describe -- perhaps even bedwetting.

People aren't any more alike on the inside than they are on the outside, and this includes the nutrients they need and the way they handle them, For an eye-opening book on the subject, read "How to Improve Your Child's Behavior Through Diet" by Laura J. Stevens and Rosemary B. Stoner (doubleday & Co., $9.95). The book does a good job of explaining how one food can be great for 1,000 children and poison to the 1,001st attacking one part of the body or the central nervous system, or both. It also shows you how to track food allergies and gives recipes that avoid them.

There are many other possibilities, including a low-grade infection, or even pinworms -- an airborne parasite that democratically attacks the rich and the poor. They cause intestinal cramps, irritability, rectal itching and poor sleep.

Keep a close record of your child's teperature, his moods, his foods, and his environment, especially if you have added anything new in November. This will help the pediatrician make thoghtful diagnosis, just as much as the urinalysis and book workup he will order.

So take off the hair shirt, love, have a fancy dinner with your husband after the baby has gone to sleep and congratulate yourself on all the things you've been doing right. Most of us, you know, never get little children to eat vegetables at all.