sink or swim
sink or swim {v. phr.}
To succeed or fail by your own efforts, without help or interference from anyone else; fail if you don't work hard to succeed.
When Joe was fourteen, his parents died, and he was left by himself to sink or swim.
Tom's new job was confusing and no one had time to help him learn, so he had to sink or swim.
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Source: A Dictionary of American Idioms
