Thank goodness they stuck it out!
Hemsworth was sick of dealing with rejection and decided to go home to Australia. However, he first auditioned for The Cabin in the Woods, gaining the part of Curt in 2009. Two years later, he'd play Thor in the MCU.
Like Melissa MCarthy, Hamm had promised himself if he didn't make it by 30, he'd quit. He ended up turning 30 on the set of We Were Soldiers and says that he "made it under the wire."
Harbour was ready to give up on taking film acting seriously when he was cast in Stranger Things, saying that he would do it to make money on the side while pursuing theater but wouldn't put his heart in it. He thought he'd never get the Stranger Things role, and after he got it, he decided that it would be the last film or TV role he cared about. It ended up reinvigorating him and encouraging him to pursue more film and TV acting. This year, he'll star alongside Scarlett Johansson in Black Widow as Marvel character the Red Guardian.
After his brother was murdered in 2008, Ruffalo got rid of his agent and manager and moved to upstate New York with his family, saying he'd "had it with acting." He ended up taking a role in The Kids Are All Right, and this helped him fall in love with acting again, especially when he saw it at the Sundance Film Festival.