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Bite off more than you can chew

It means to take on more responsibility or work than you can actually handle. 

Examples:

1- I told my boss I could finish three reports by Friday, but now I’m staying up all night. I definitely bit off more than I can chew.

2- He thought learning two languages and playing in a band during his first year of college would be easy. He totally bit off more than he could chew.