bite off more than you can chew

To bite off more than you can chew means to take on more responsibilities than you can handle. The cliche is often used in reference to people who to agree a variety of tasks or take on a large number of responsibilities and who honestly think they can handle, but then it turns out they took on too much and are unable to be successful in all of these tasks. It is often a warning to people who agree to do more than they can realistically handle.