After a burst of enthusiasm in the first weeks of the Reagan administration, Americans appear to be turning pessimistic about the future of the nation's economy, according to the findings of a nationwide Washington Post ABC News poll.

The public is sharply divided on whether President Reagan's economic recovery program will help bring an end to inflation, with 49 percent saying it will and 41 percent saying it will not. Only one month earlier, an overwhelming majority -- 64 to 28 percent -- felt the Reagan program would help end inflation.

In addition, the Post-ABC News survey strongly suggests that a great many Americans are coming to believe that their own personal standards of living have been seriously damaged by persistent inflation, and that conditions are not going to get better for them or their children. The public's fear of inflation, which many economists think is an important factor behind the nation's financial ills, appears to be getting worse instead of better.

In all, according to the poll, 3 in 10 Americans report making major sacrafices in their standards of living because of inflation, and another 6 in 10 say they are making some sacrafices. Only 1 in 10 say they are not making any sacrifices.

Even as they confirm substantial cutbacks in the way they live, many Americans continue to buy, buy, buy in order to beat inflation. Question: "These days, do you often find yourself buying things sooner than you would otherwise, because you know prices are only going to go higher?" Forty-one percent say they do.

This inflation psychology, should it continue, could have major impact on the outcome of Reagan's economic recovery program. For one thing, Reagan and his chief advisers expect that congressional passage of his program will inspire a major shift in public expectations, a new confidence that double-digit inflation can be ended. That, in turn, is supposed to stimulate investment and real growth.

For another, any continued decline in public support for the Reagan program could affect the way Congress deals with the president's controversial calls for massive spending cuts in social programs and a 10 percent reduction across the board in federal income taxes in each of the next three years.

Some political observers have maintained that congressional opponents of the Reagan program have been reluctant to attack it until now only because it had gathered immediate public approval. On Tuesday, House Speaker Thomas P. (Tip) O'Neill Jr. (D-Mass.) said that his mail from the public, after initially running "1,000 to 1 for Reagan," had turned against the Reagan budget proposals.

The Post-ABC poll was conducted from March 25 to 29, the days just preceding the assassination attempt of Reagan. Its findings parallel those of a Gallup poll taken shortly before then which showed the president receiving an overall negative rating for his handling of the nation's energy problem, a comparatively low rating for his handling of unemployment, and sings of a falloff in his previously high standing for coping with the nation's economic woes.

Neither Gallup nor the Post-ABC poll show Reagan's progam in trouble with the public. Almost two-thirds of those interviewed by the POST-ABC News survey say they approve the president's spending and tax cut proposals. Nevertheless, support has declined. In a Post-ABC poll in late February, just after Reagan described his plan in a televised address to Congress, almost 75 percent said they approved the spending and tax cuts.

In addition, the new poll shows an increased number of Americans who perceive the spending cuts as hurting the poor and the tax cuts as benefitting the rich, and an emerging sense that Reagan wants things that way. bIn February, for example, 64 percent said Reagan "cares equally about serving all the people," while 23 percent said he cares more about serving upper-income people. In the new poll, 54 percent say Reagan cares equally about all people and 34 percent say he cares more about serving upper-income people.

One leading public opinion expert, Robert Teeter, said the Post-ABC findings semed to be "a reflection of the first specifics coming out of the Reagan program." Teeter said that in his own polling he has found a split in views over the thrust of the Reagan program and particular aspects of it.

"If we ask, for instance, do you support the entire program, the answer is yes -- but not for specific programs," Teeter said.

Teeter, whose clients include many leading Republican officeholders, said his own surveys have uncovered the same disquiet about the future as that found in the Post-ABC poll. "There is a real difference if you take this period compared to some other postwar recession period. Inflation has become so severe that instead of saying that things are bad in the country, people are saying, 'Man, I'm hurting, myself.'

Three-quarters of the people are going through life thinking about how the economic problems are affecting them. That's a hell of a lot different then we've had in 20 or 30 years," Teeter said.

It is beyond doubt that many Americans are hurting substantially. In the Post-ABC poll, 20 percent say it is very difficult or impossible to meet their monthly bills and 37 percent say they have been going deeper in debt to make ends meet.

Seventeen percent say they have found it necessary to skip meals to save money, 20 percent say they are eating worse than they did a year ago, and 69 percent say they are spending a greater portion of their income on food than they did a year ago.

Many of those who report making purchases out of concern that prices will only get higher are, at the same time, making extreme cutbacks wherever possible in the way they live. Basic attitudes and ways of life are undergoing sometimes drastic change.

One series of Post-ABC questions asked people whether they were making any of a wide array of specific changes to cope with inflation, such as spending more free times at home instead of going out. Fifty-seven percent said they are staying home to save money as much as they can, and another 25 percent said they are staying at home somewhat more than they used to. Only 16 percent said they have made no such change.

Along the same line, 29 percent say they are shopping at second-hand stores as much as they can and another 20 percent say they have taken to buying at such stores somewhat. More than 7 in 10 people say they are not buying clothes the way they used to; 60 percent say they have cut back on donations to charity.

"Inflation has kept me and my husband working and doing all the overtime we can," said a 31-year-old Michigan woman who described her job as "sweeping floors with a power sweeper in a manufacturing plant." The household's combined income is now in the $20,000-to-$30,000 range. The mother of two young children, she blames inflation for having taken from her "time to spend with my family."

Another factory worker, a welder who lives in a suburb of Houston, reports that he and his wife have a combined income of more than $30,000. But, he says, inflation "caused my family to move away from our home state in order for my wife and myself to find better employment. . . . It has also caused our standard of living to go down."

The fear of future inflation seems far more pronounced today than it did in late 1979, when inflation was an equally acute national problem. It seems to be dimming as essential element of the American dream -- the idea that, over time, life moves onward and upward, that you will be better off than your parents were and that your children will be better off than you are.

In November 1979, a Washington Post poll asked people whether they felt they were better off financially than their parents were at the same age. A total of 81 percent said they were, and 16 percent said they were not. In the new Post-ABC News poll, only 64 percent say they are better off than their parents were, and 35 percent say they aren't.

The 1979 survey asked people whether they thought their children would be better off financially in the years to come than they themselves were. The answer was a resounding yes, with only about 20 percent saying their children would not be better off. Today more than twice as many people, 43 percent, believe their children will not be better off. 1,305 Phone Calls March 25-29

A total of 1,305 people nationwide were interviewed by telephone March 25 to March 29 in the Washington Post-ABC News poll on how Americans are coping with inflation. Theoretically, figures based on that many interviews are subject to a sampling error of about 3 percent in either direction, 95 percent of the time.

The latest available U.S. Census figures on age, sex, education and race were used to adjust the sample slightly so that it matches the overall population in those characteristics.