Americans who have benefited from the Affordable Care Act are feeling some relief at the failure of Republican efforts to repeal it, but they face new anxieties with President Trump tweeting that “ObamaCare will explode.” Premiums have risen and major insurers have backed out of the state markets where people can buy insurance online under Obama’s signature health care law. But people who say it saved their lives or helped them start a business want lawmakers to fix these problems, not encourage them. Republican promises to repeal and replace the law foundered on Friday when House Speaker Paul Ryan abruptly pulled the party’s health care bill to avoid almost certain defeat.