Under fierce pressure from forces beyond his control, Jimmy Carter has been forced to make the most painful retreat of his young presidency. The consequences of that retreat will be substantial.

Some - particularly in the short term - will be positive for Carter. He has a monkey off his back now, even if he has lost his closest friend in Washington.

Carter's presidency has been growing into a multi-ring spectable, with events going on simultaneously all over town. But as long as Bert Lance hung on, the spotlight was elsewhere - the rigmaster's show was all but ignored.

The Lance affair largely dissipated the impact of the grand ceremony Carter arranged for the signing of the new Panama Canal treaties.Lance's troubles have obscured major struggles over the President's energy program, the dispute over refinancing the Social Security program, several important diplomatic exchanges and more besides.

Lance's resignation will bring the audience back into the main tent, which is bound to help Carter in the coming weeks and months. But the Lance affair has altered the audience's perceptions, probably permanently, and the effects of that change remain to be seen.

No one can accurately measure the impact on a President of an unsuccessful public attempt to impose the presidential will. Carter invested a piece of himself in the Lance affair: "Bert, I'm proud of you" is likely to ring in his ears for a long time.

Carter refused persistently yesterday to acknowlege that the accusations made by Lance's critics had any merit, but he acknowledge during his press conference that their criticisms would take a toll.

"I would guess to some degree an unpleasant situation like this would be damaging somewhat," Carter said. Publicity about the Lance affair "creates doubt - about the integrity of me and our government," he observed.

Many members of Congress returned from their summer vacations with the impression that their constituents were reevaluating Carter's "Mr. Clean" image in light of the Lance affair.Polls show the same thing.

Carter's more immediate audience in Washington has watched with fascination over recent weeks - watched what now amounts to a defeat for the President. Congress, the press, the political community in this cynical town all have seen the city's No. 1 citizen in a new posture.

Carter obviously detested the prospect, but his hand was forced by outsiders - by many of the entrenched forces against whom he campaigned so eloquently last year. At his press conference yesterday the President refused to acknowledge in any way that he had been a loser in the Lance affair, but despite all that, he lost.

It wasn't the first time. Carter lost his first choice to head the Central Intelligence Agency, Theodore C. Sorensen, and he has lost more than once on Capitol Hill. But Lance was a member of the President's innermost circle. IT was more than another lost political skirmish for Carter to let him go.

A serious problem now arises with those elements in Washington and the country to whom Lance had become a symbol - a symbol of soundheaded conservatism within an administration heavily populated by people these elements regard as dangerously liberal or even radical.

Lance was Mr. Balanced-Budget-by-1981, the friend of business, the enemy of big spending and big government. Carter decided early on that he had to do what he could to sustain and encourage "business confidence." Lance was his principal agent in that effort.

Lance had been an effective salesman. Business did like him - until he got in trouble, when the American Bankers Association and many others dropped him like a contaminated potato. Many conservatives on Capitol Hill like him too, as Sen. Ted Stevens (R-Alaska) demostrated during Lance's appearance last week before the Senate Committee on Governmental Affairs.

Stevens led Lance through an embarrassing series of questions about the way he financed his 1974 gubernatorial campaign, but when he was finished he took the hearing room by surprise. Instead of criticizing Lance, he wished him well, hoped he would "survive" and succeed in balancing the budget.

Speculation will now arise as to who Carter might find to replace Lance as his emissary to the conservative and of the political spectrum. There has already been talk in the White House that former Democratic National Chairman Robert Strauss might be the one - a lawyer, but one who had lots of corporate clients.

Lances experience in the public eye here might well discourage - as he himself repeatedly suggested - other businessmen from volunteering to take his place in Washington.

"I'll tell you the truth," said one New York businessman who considers himself a liberal Democrat. "I would not take that job, even if I was qualified for it. I couldn't take that scrutiny."

Carter may well have to answer for his actions in the Lance affair. Were he and his Georgia intimates too narrow-minded, did they see only the struggle involved, and not the issues of ethics and morality that apparently struck many citizens? Did their emotional attachment to Bert Lance or their lack of experience make them ineffective political operators?

Carter may also have to answer again for his insistent endorsement of Lance's business ethics, which remain under official scrutiny.