"I've been wondering why I'm killing myself and wondering why the Democrats caved in to George Bush," Sheehan told The Associated Press while driving from her property in Crawford to the airport, where she planned to return to her native California.
In what she described as a "resignation letter," Sheehan wrote in her online diary on the Daily Kos blog: "Good-bye America ... you are not the country that I love and I finally realized no matter how much I sacrifice, I can't make you be that country unless you want it.
"It's up to you now."
Sheehan began a grassroots peace movement in August 2005 when she camped outside Bush's Crawford ranch for 26 days, demanding to talk with the president about her son's death in Baghdad in 2004.
Over the next two years, she drew huge crowds as she spoke at protest events. But she also drew criticism for some actions, such as meeting with Hugo Chavez, Venezuela's leftist president.
"I have endured a lot of smear and hatred since Casey was killed and especially since I became the so-called "Face" of the American anti-war movement," Sheehan wrote in the diary.
Kristinn Taylor, spokesman for FreeRepublic.com, which has held pro-troop rallies and counter-protests of anti-war demonstrations, said dwindling crowds at Sheehan's Crawford protests since her initial vigil may have led to her decision. But he also said he hopes she will now be able to heal.
"Her politics have hurt a lot of people, including the troops and their families, but most of us who support the war on terror understand she is hurt very deeply," Taylor said Tuesday. "Those she got involved with in the anti-war movement realize it was to their benefit to keep her in that stage of anger."
When Sheehan first took on Bush, she was a darling of the liberal left. "However, when I started to hold the Democratic Party to the same standards that I held the Republican Party, support for my cause started to erode and the 'left' started labeling me with the same slurs that the right used," she wrote in the diary.
She said she sacrificed a 29-year marriage and endured threats to put all her energy into stopping the war. What she found, she wrote, was a movement "that often puts personal egos above peace and human life."
She said the most devastating conclusion she had reached "was that Casey did indeed die for nothing ... killed by his own country which is beholden to and run by a war machine that even controls what we think."
Sheehan told the AP that she had considered leaving the peace movement since last summer while recovering from surgery.
She decided on Memorial Day to step down and spend more time with her three other children. She said she was returning to California on Tuesday because it was Casey's birthday. He would have been 28.
"We've accomplished as much here as we're going to," Sheehan said, saying she was leaving to change course. "When we come back, it definitely won't be with the peace movement with marches, with rallies and with protests. It will be more humanitarian efforts."
Last year, with $52,500 in insurance money she received after her son's death, Sheehan bought 5 acres near downtown Crawford as a permanent site for protests.
"Camp Casey has served its purpose," she wrote in the diary. "It's for sale. Anyone want to buy five beautiful acres in Crawford, Texas?"