(CNN) -- With no way to counter toxic jellyfish that brought intense pain and partial paralysis during an attempted 103-mile ocean crossing from Cuba to Florida, endurance swimmer Diana Nyad said Monday that she will not make another attempt at the feat.
"I can't beat those guys. They're too much for me," Nyad said.
Nyad, 62, swam 82 nautical miles of the 103-mile crossing before two stings from what her team identified as potentially lethal box jellyfish forced her out of the water Sunday morning.
In a frequently emotional news conference in which she removed her shirt to reveal sting marks, Nyad said she initially planned to muscle through the intense pain. But she said she eventually had to give up as the toxins began to cause partial paralysis and made it increasingly difficult for her to breathe.
Nyad still had a wheeze Monday, said one of her doctors, Clifton Page of the University of Miami.
Toxins from the stings also affected members of her crew trying to help her.
"This was a life-threatening situation we were in," Page said.
Nyad stopped the swim Sunday morning after 40 hours in the water.
"It's not easy for me to let go of this dream, and I'm in distress about it," she said.
She said it was "naive" of her not to anticipate problems from the jellyfish, which she said are proliferating throughout the world's oceans because of climate change. Unlike sharks, which her team countered with divers and electronic devices, no good tools exist to fend off jellyfish, Nyad said.
She said she knows she could complete the crossing were it not for the creatures. She said the last two years of training were not wasted.
"It's been a grand, elevating, life-confirming experience these last two years," she said.
Nyad began her attempt Friday evening from Havana's Hemingway Marina, expecting to spend about 60 hours in the water.
It was her third attempt to complete the swim.
Her first, in 1978, was brought to an end by strong currents and bad weather after almost 42 hours in the water.
She made a second try in August, but she was pulled from the water after 60 miles and almost 29 hours of swimming. She blamed a shoulder injury she suffered early in the journey and an 11-hour-long asthma attack.
She said people inspired by her efforts should continue to train their bodies, test their wills and dream big. And, said said, not being able finish the crossing against such odds was no sign of failure, for her or her team.
"There's so much boldness in living life this way, and we did it all, and no one can take it away from us," she said.