Leonard Bernstein was in a reflective mood over a late dinner in Philadelphia Wednesday night, explaining his recent decision to devote all of 1980 to composing -- whih caused him to cancel his guest conducting appearance for the final four weeks of the National Symphony's coming season.

"I don't know what I am goint to write," he began, "but I can't sit down and compose in four weeks these days. When I was a kid I could do it. These days it takes one whole week getting Beethoven and Mahler out of my system before I can get started."

Following reports that he had already begun work on an operatic version of Vladimir Nobokov's novel "Lolita," Bernstein concedes that "there is so much pressure to write an opera, but I don't know. I really do not know what I am going to write."

But he does know for certain that as soon as the composing year is over, he will begin a major movie project with Italian director Franco Zeffirelli.

Among the detailed plans and engagements stretching ahead for the next four or five years is the recording and filming of a complete, uncut version of Wagner's "Tristan and Isolde," with Zeffirelli as director and a cost headed by Hildegarde Behrens and Peter Hoffman in the title roles.

"We are going to do it one act at a time, taking a total of nine weeks for the whole thing, and with time out between each act." Bernstein's enthusiasm lights up his face while he gestures in delight at the prospect.

"Doing it this way there is plenty of time to prepare everything. And you do not have to be afraid that the tenor will be dead by the time you get to the third act." He stopped for a moment, then added in a tone that held a kind of inner satisfaction, "It's interesting that we got into this at the same time that Karajam and Carlos Kleiber were thinking about doing 'Tristan' too."

Bernstein said he made his decision to stop conducting in 1980 "while I was on the Japan tour with the Philharmonic." He led the New York Philharmonic's recent five-weeks tour while still suffering from the effects of a severe case of amoebic dysentery he picked up when he went to Mexico City with President and Mrs. Carter last February.

"I had a rough time last year," Bernstein said, "with Felicia, and then all the cancellations at that time." His wife, actress Felicia Montealegre, died in June 1978. His first major public appearance after her death was last Aug. 25 when National Symphony Music Director Mstislav Rostropovich and myraid Bernstein friends gave him a gala 60th birthday party at Wolf Trap.

"I've spent too much time conducting," said the former music director (now laureate conductor) of the New York Philharmonic and a man whose conducting services are constantly sought years in advance by the world's major orchestras.

"I feel I have to conduct. Just last week at Tanglewood, it was not only that the Boston Symphony was wonderful. But the kids in the student orchestra were marvelous. I love working with them. "maybe I should do that every year. They have asked me to."

Looking tan and healthy -- the swimming pool is his favorite summer recreation after a hot outdoor rehearsal Bernstein said that "for a while I felt old. But these last few weeks, I have felt young again and vigorous."

It's obvious that this astonishing composer-conductor-lecturer-author and TV personality is feeling vigorous and then some. Last week at Tanglewood he conducted the Boston Symphony in the Ninth Symphony by Gustav Mahler, the same work he conducted last night with the Philadelphia Orchestra at Robin Hood Dell. He will conduct it again in Berlin with the Berlin Philharmonic in early October. "That concert will be a benefit for Amnesty International." he said. "That is the only circumstance under which I would conduct the Berlin Philharmonic."

In addition to his National Symphony engagement, Bernstein has canceled appearances in Salzburg, Austria, in connection with the 100th anniversary well as at the Berlin Festival and the New York Philharmonic, among others.

Bernstein's cancellations, which are specifically for the calender year 1980, will not interfere with his appearances at the Kennedy Center in late October and early November, when he will conduct the Vienna State Opera in performances of Beethoven's "Fidelio" and the Vienna Philharmonic in two concerts -- including a concert version of the second at of "Tristan."

"I'm sure I will come to conduct the National Symphony again," he said. "I certainly hope so."

For all his enthusiasm in talking about conducting and filming an entire "tristan" -- and about his yearlong beak to compose -- Bernstein seems no nearer a satisfactory solution to his real dilemma. He is a composer-conductor. And he really must do both. CAPTION: Picture, Leonard Bernstein, by Craig Herndon