IT WAS the morning after her unforgettable gala, the night that may well have been the biggest, fondest farewell bash in operatic history. On that blustery autumn day last week, Beverly Sills was sitting in the swirling turbulence of the small office where she operates as general director of the New York City Opera, reflecting, as if in a golden mirror, over a long and glorious career.

Asked how she felt, about the night before, she began with that big, famous laugh: "It was the way I wanted it. I wanted to go out on an upper. fThank God it's over." A reflective smile crossed that face that has been the face of Elizabeth of England and Anna Bolena and Maria Stuarda and Manon and Cleopatra.

That morning, Sills got up "at a quarter to seven. For coffee and French toast, you know. Peter [Greenough, her husband] kind of buries himself in the paper in the morning and walks the dog. and Muffy [her daughter] and I have our mama-daughter time. We get our best work done. I put the coffee on and we were talking, and she said, 'Mama, how do you feel today, you feel sad?'

"I said, 'No, I feel wonderful.' 'Yes, but how do you really feel?' And I said, 'Muffy, what's written on Martin Luther King's tombstone? She smiled, and she said, 'Free at last, free at last, thank God Almighty, I'm free at last.'"

But is it freedom, at 51, to be responsible for running one of the world's busiest opera companies for one of the most discreminating audiences?

"Yes," Sills answered. "I'm free of one kind of discipline that simply doesn't blend with the discipline of this job of general director. Because there were . . ." She paused. "Several things happened to me this last year. First of all, the coach I had been with for 19 years, Roland Gagnon, died.

"I brought him into the company to work with me as music administrator, and he died in his sleep. It was just tragic. He used to come over to the house every morning, five days a week -- rain, snow, slush. I would be in my robe, dying, sometimes even without my teeth brushed. He would be at the piano, with a huge mug of coffee, pushing me on, pushing, always with interesting repertoire for me to learn. I used to pretend sometimes to be very irritated, it was so early, but actually it was a way of life for me, and I adored him.

"Once he died, and that discipline left me, I really had no desire to find anyone else. I made a few stabs at getting up at 9 o'clock, doing it myself -- boring! Just couldn't handle it.

"And so in many respects -- apart from the idea that the time was right for me to go -- I was already making compromises in my repertoire, already making vocal compromises. I am used to being able to tell my voice exactly what to do and if occasionally it didn't do what I wanted, I knew the next time it would. If I had an off night I knew the next night was not going to be one. And now it became unreliable. I found that very offensive. I found myself thinking of my voice for the first time as a separate entity. You know -- when it's not behaving. And I decided I could not spend the rest of my life doing recitals by adjusting the repertoire, or doing all this cream puff I was doing: Rosalinda, Adele, Rosina, Norina. This was a voice that was used to doing Elviras and Elizabettas and Maria Stuardas. I couldn't go on doing repertoire like those others. It's not me.

"And I thought, What the hell! I didn't enter this as a marathon to see if I could last longer than anyone else. That wasn't what the career was about. Years ago, 1967 or '68, when I was on the cover of one of those national magazines, I said I would rather have 10 or 12 exciting years than 15 or 20 bland ones. I've had them. There's no question that the repertoire I chose took its toll of my voice. I could have wrapped it in cotton and maybe gone on four or five years more, but that's not what I wanted. So you see . . ." And the smile broadened into a wide grin. "I go out cheerful because the choice wasn't made for me. I made it.

"My dad used to say to me that there is really no great success without a big element of risk. And you know, it's music to my ears when I look back on the roars that came after I sang. Terrific!That was one of the reasons I felt that the time was right.

"The other: The opportunity to stay with this company. It's a unique company. You know, I was offered San Francisco. [Director Kurt] Adler was right there at the luncheon when they talked to me about it. And I have a lot of friends out there. But I didn't want to direct that kind of a company. And this is home -- the people who sing here are not only my former colleagues, they are my friends. I'm very excited about it. I think with a little bit of luck I can make this the place. I have the feeling.

"People say, 'Don't you have any regrets?' Until this past June, I would have said no. Oh, I mean, I would have loved to sing Butterfly. I would have been the biggest Butterfly in capitivity. I would have loved to have done Salome, but I didn't happen to have the voice. But that's not a true regret. But I was in Munich. And Carreras was singing Werther, and I suddenly found myself just sobbing, and a subconscious thought passed through me, and I thought, 'O my God, I wish I had done Charlotte'!"

For the future of her company, Sills said, "I want to do a lot of French repertoire. I'm bringing in a new 'Cendrillon,' 'Mignon,' 'Dardanus' of Rameau -- the more of that the better. Of course 'Hoffman' will be back in the repertoire." A wry smile lighted up the face that made Manon infinitely touching: I don't know if 'Mannon' will take another revival -- the sets are sort of tired. Kurt Adler has a similar set, I might be able to do a little business with him. 'Pelleas,' with von Stade, and I'll bring back 'Faust,' of course. And right now we have 'Pecheurs de Perles.' In it we have a soprano by the name of Soviero who did one of the most exquisite Leilas, I'm open to all suggestions.

"And we have this little early Verdi festival, with 'Attila,' 'Nabucco,' and 'I Lombardi.' Ashley Putman is going to do quite a few interesting things for us. This spring I'm letting her do 'Mary Queen of Scots' [by Thea Musgrave] and 'Maria Stuarda' side by side. It will be interesting for the public to see the two Scots. And she's going to do 'I Lombardi' for us. I think it's just about right for her.

"I'm going to think more about commissioning a full-length opera," she said. "Our trilogy was a good try [the three new one-act operas commissioned and recently premiered by the New York City company: 'Madame Adare' by Stanley Silverman; 'Before Breakfast' by Thomas Pasatieri; and 'The Student from Salamanca' by Jan Bach]. The Silverman should have been much stronger than it was.

"i can't make up my mind from the audience reaction whether atonal music offends them, whether they consider it passe, or whether there's a need for more. They'll stop me in the promenade and say, 'Oh, that noise! I'm so disappointed. How can you put that on!?' Another one will stop me and say, 'My goodness, so old-fashioned, so derivative!"

"I've lived through Luigi Nono. [Sills sang in Sarah Caldwell's U.S. premiere of Nono's 'Intolleranza' in Boston years ago.] What's wrong with something that touches you emotionally and makes you weep? Gian Carlo Menotti used to get so upset when it was written about him that he was derivative from Puccini. I said 'In this world, that's a great compliment, that deserves a medal.' I'm talking to Stephen Sondheim. I made the announcement that I wanted to bring 'Sweeney Todd' into this company and cast it with our opera singers. The Met couldn't do it, but we could."

Sills is enthusiastic about her singers, both established and new. "For basses, I'm very lucky. I have Justino Diaz, Sam Ramey and Robert Hale. And two very good ones coming up: Seabury and Martinovich."

The New York City Opera season is long, but Sills would like to lengthen it still more, and she knows just where and when and with what.

"We can't make the fall and spring season longer because the ballet comes in. I would like to take the theater in the summer -- we're in negotiation for '82. By that time we will have our 'Student Prince' -- it played to a 99.6 percent capacity, which is indicative of the public demand for it -- next year we're goig to do a 'Great Waltz.' By the time '82 comes, I hope to have a new production of the 'Merry Widow,' and then we will do a fourth, new one. It could be 'The Sound of Music' or 'The Song of Norway,' which we're looking at.

"Our fall season lasts from before Labor Day to about the 12th of November. Then we open for the spring again about the 19th of February and close generally by May 1. So we have about 86 performances fall and spring, that's 170, and we have almost three weeks in Los Angeles. And we will have two weeks in Wolf Trap in '81."

Washington, like the rest of the world, has seen and heard Beverly Sills sing for the last time, live. But the good news is that she and her company are going to be around for years to come.