You use the expression ‘Bite off More than You Can Chew’ to indicate that you’ve
taken on too much.
Example of use: “Be careful not to bite off more than you can chew. You’re going to have to take a day off sometime!”
The origin of the phrase ‘bite off more than you can chew’ is an allusion to taking a mouthful of food that is too large to handle comfortably. While cautions against taking on more than you can handle appear in medieval sources, the metaphor itself dates to about 1870.