Richard Nixon tells us he was terribly tired, drained emotionally, mentally, physically, when he stepped aboard that helicopter to leave the White House the last time in disgrace. He remembers hearing his wife murmur, "It's so sad, it's so sad."

What may be the saddest commentary of all is that Nixon, even at this late date, still doesn't seem to understand that millions and millions of his fellow citizens were equally exhausted - and saddened - by the strain of his long fall from grace. His own description of those events, as rendered in the concluding scheduled interview last night with David Frost, is almost entirely preoccupied with self.

It's a cheerless, pitying, personal portrait that Nixon sketches. He comes over as bitter, resentful, vengeful, nursing stale grudges, baring old wounds, shouting at the fates, rationalizing his actions, fancying plots and conspiracies, assailing his perceived enemies and laying into them one by one. The liberals. The press. The Communists. Woodward and Bernstein. Elliot Richardson.

In the history of the presidency there probably has been nothing so personally revealing. It's almost too personal, and painful, to watch, particularly the closing segment, when he talks about "almost unbearable." He knows what living a life without purpose, a life it means, he says calmly "not having anything to live for." And then adds:

"No one in the world, no one in our history could know how I felt. No one can know how it feels to resign the presidency of the United States."

Last night's fourth interview with Frost was billed as dealing with Nixon's final days, but it takes in much more ground than that. It covers the overthrow of the Allende regime in Chile, the Spiro Agnew case, the problems over his income taxes, the events of his closing days and resignation and the acceptance of his own pardon. And there may be yet another Frost-Nixon interview shown later this summer.

In a sense, last night's interview finally closes the door on Watergate. It comes two days after the Supreme Court has turned down final appeals by three of the convicted Watergate principals, John N. Mitchell, H. R. Haldeman and John D. Ehrlichman. And yesterday the Justice Department announced it was abolishing the Watergate special prosecutor's office.

Now Nixon has given, as he says, his side of his case.

Every segment of the interview provides new insights into his thinking - and evokes new controversies over his version of events. But it's the tone of bitterness that leaves the strongest impression.

In talking about his wife's stroke, he mentions that it came three days after she read the Woodward and Bernstein book, "The Final Days."

"I didn't want her to read it because I knew the kind of trash it was and the kind of trash they are," he says.

He goes on, without prompting from Frost, to attack all the media. The Supreme Court, he says, has given them "a license to lie." Then:

"And, so my point is - let's just not have all this sanctimonious business about the poor repressed press. I went through it all the years I've been in public life, and . . . they never have been repressed as far as I'm concerned. I don't want 'em repressed, believe me, when they take me on, or when they take any public figure on, Democrat or Republican, liberal or conservative, I think the public figure ought to come back and crack 'em right in the puss."

Throughout, Nixon keeps interjecting a note of self-pity.

On his tax problems over deducting his personal papers: "I just didn't frankly, have any good legal adivce as to the fact that they were deductible."

On denying he has any foreign bank accounts: "I hope I have the opportunity sometime to maybe write a check in a foreign land on my bank account in San Clemente."

On trying, unsuccessfully, to switch the line of questioning, causing him to dab at his face with a handkerchief: "Oh, I can take this as long as you want."

He also continually employs a familiar Nixon technique. He shifts the blame, or responsibility, to someone else. It was an Italian businessman, anonymous, who convinced him in his Oval Office that Chile posed a potential Communist threat - the southern half of a "red sandwich," the northern part being Cuba. It was Lyndon Johnson who talked him into taking those tax deductions on his papers. It was his personal lawyer, Jack Miller, who persuaded him to accept his pardon because he wouldn't be able to get a fair trial.

And, in a rather stunning turnaround, it was not Henry Kissinger who was trying to comfort Richard Nixon in that celebrated final night of praying and crying in the White House. It was Nixon who was the comforter, Nixon who was trying to stop Kissinger from resigning: "Henry, you're not going to resign. Don't ever talk that way again . . . The country needs you. Jerry Ford needs you and you have got to stay and continue the work that we have begun."

Probably the single most revealing episode in the interview concerns Nixon's account of the Agnew case. He disparages Elliot L. Richardson, then his Attorney General, for his political and personal motivations. It was no secret that Richardson and Agnew didn't like each other, he says, no secret that Richardson wanted to be President or Vice President.

After Nixon orders an investigation by Henry Petersen, in charge of the Justice Department's Criminal Division, he's told strongly by both Petersen and Richardson that Agnew's case is too serious to be handled by a fine.It would require a prison sentence.

Nixon, the nation's chief law enforcement officer and a lawyer himself, reacts to that news by attempting to steer the case away from the courts. It's Nixon who suggests Agnew's case should be handled like a presidential impeachment, Nixon who tries unsuccessfully to persuade then-Congressman Gerald R. Ford to lead that effort. He never once expresses any sense of outrage over the criminal case, nor of a betrayal of trust by the second-ranking officer in the land, someone he personally had chosen twice for high responsibility.

When Agnew offers him his own rationale - what he had done was "common practice in most of the Eastern states and many of the Southern states" - Nixon again draws no judgment, legal or moral. Asked by Frost which version he leaned toward, Agnew's or Petersen's, the former President answers:

"I was very pragmatic. In my view, it really didn't make any difference."

Nixon then goes on to equate Agnew's case with his own: "I felt that in his heart he was a decent man. He was an honest man. He was a courageous man. He made mistakes; I made mistakes."

Then his personal bitterness seeps out. There was a double standard over Agnew: "because he was conservative, because he was one who took on the press, he got a lot rougher treatment than would have been the case had he been one of the liberals' favorite pin-up boys."

At the end, Nixon offers a poignant and painful soliloquy. No, he didn't have a death wish. No, he didn't think in suicidal terms. That was bunk.

But, he quickly says, "I felt that resignation meant that I would be in a position of not having really anything to live for, and related to the fact that it is life without purpose, not having anything to live for, that it could be a very, very shattering experience."

Richard Nixon then acknowledges that his resignation has been a shattering experience, and to a certain extent it still is. He might have said a shattering experience for him, and for all of us.