In 1963, Pauline Collins was a young and struggling actress in Killarney, Ireland, working with a close-knit repertory theater group. She was 23 years old, single and in love with life`s possibilities when she discovered she was pregnant.

Suddenly everything changed. After much anguish and self-denial, she decided that offering the child for adoption was the best choice. It is a decision that led to doubt, guilt and more anguish.

Collins-the English actress who won the 1989 Tony award for her portrayal of the title role in ''Shirley Valentine''-recalls the phases she went through during that tumultuous year: the disbelief and anger when she learned she was pregnant; the denial and absolute refusal to alter her lifestyle; the subsequent feeling of despair and abject loneliness and thinking that if she ignored the situation her problems would go away; and the final realization that she must accept the consequences of her behavior.

Collins gave birth to a daughter, Louise, in a home for single mothers that was run by nuns. She spent six weeks there getting to know her infant daughter until the moment came when it was time for Louise to go to her new home.

Later, Collins would think of her daughter and ask, ''Why did I do it?

Why did I give you away?'' And now, nearly 30 years later, Collins addresses such soul-searching questions in her recently published book ''Letter to Louise: A Loving Memoir to the Daughter I Gave Up for Adoption More than Twenty-five Years Ago'' (HarperCollins, $20).

The book is just what the title implies-a personal and heartfelt letter from a mother to a daughter.

Collins, who was reunited with her daughter six years ago, writes in her book about deep, personal loss.

She also writes about choices, about the consequences of those choices and about the stigma of being a single parent who bore a child out of wedlock in the days before social service agencies and women`s self-help organizations made other alternatives possible.

Ultimately, her book is about growing up and coming to terms with painful decisions made at a young age. (''In the end only I could take responsibility for our lives, yours and mine,'' Collins writes.)

Collins, 52, who also won an Oscar nomination for the film version of

''Shirley Valentine'' (the story of a bored Liverpool housewife who chucks her prosaic life to start over again on a Greek island), left the glamor and fantasy of theater and films behind during a recent visit to Chicago.

Despite life spent under the media`s watchful eye-she is a well-known figure in England-Collins comes across as a private person, not the type to spill her secrets.

Why then did she come forward, and why so many years later?

''The story broke in the English press, which I suppose was inevitable,'' she said in an interview.

''So it was flashed all over the papers for a while,'' she said. ''Six months later I was approached by someone who said they were writing my story

(that is, not a life of Pauline Collins but a book on the adoption).''

Collins was asked if she would cooperate with the project.

Two reporters in particular were interested, Collins said-''one more serious than the other. She (the reporter) was very nice but I didn`t want it to be sensationalized. It`s such a precious thing to me. It`s so personal. And I don`t think even I knew how personal it would be until I started doing it.'' Collins feared losing her ''heart.''

''I thought `Well, anybody can write about you,` '' she said. ''The best defense against it is to do it myself.''

''Letter to Louise'' is her first attempt at writing. Beginning was the most frightening aspect.

''I procrastinated an enormous amount before doing it because of the subject and because, I think, I am a procrastinator by nature,'' she said.

''But having once beaten myself into it, then I actually worked quite quickly.''

It took Collins about 10 weeks to write the text although, she said, ''It took me probably a year to get to that point, partly due to the fact that I was doing the `day job.` ''

The ''day job,'' in this case, refers to Collins` work on location in Calcutta, where she was shooting the film ''City of Joy'' with Patrick Swayze. She wrote the book between filming scenes.

''Writing it is one thing,'' she said. ''Seeing it in print is another.'' It was very important that her family agreed with her decision to go public.

''Before I even set pen to paper, I asked permission from all the people involved-the three families (Collins`, Louise`s adoptive family and that of her her birth father, Irish actor Tony Rohr) and, of course, Lou herself,''

she said.

''And they all said they thought it was a good idea to do it. When I finished, which was just Christmas-New Year of this year (1991-92), I had copies typed up from my longhand and sent round to all of them. They all read it at the same time. So I said, `What do you all think? Is it a good idea to still have it published?` ''

No one objected.

Despite the assurances, Collins still had doubts.

''Even when we were in the first-proof stage, I said to Lou, `This is your book, and if you don`t want it to be published it will remain just for you,` '' Collins said. ''She said she thought it would help people, which I didn`t at the time consider or believe to be true. But now that it is published I`m already beginning to have that kind of response, which has surprised me.''

The book is packed with anecdotes and details, insights filtered through an earthy sense of humor. Conversations are recalled. Memories are evoked, from the good times, such as the joy of her first opening night (''It was a rare kind of high, never to be repeated'') to the bad, as when the realization of her predicament finally hit home (''I didn`t want to face anyone, to speak to anyone at all'').

Collins didn`t keep a diary; she wrote the book from memory.

''I have a good memory,'' she said. ''While I was writing I found that somehow, automatically, I went back to that time. Both my mother and I have very good recall, not of everything but obviously of very particular times. Of course, our memories of childhood are very strong always. Little things which stand out. There are certain memories which are very vivid because they obviously made an impression on me at that time. You have a kind of snapshot picture of that.''

Collins said she knew that one day she would write her feelings down-what form it would take and whether or not it would be published was not a concern- to let her daughter know why she did what she did.

''It was a book that I intended to write for her,'' she said. ''I started to write it over the years-maybe that`s why it was so quick in the end-because I had in my mind what I wanted to tell her.