bite off more than you can chew
From Longman Dictionary of Contemporary Englishbite off more than you can chewbite off more than you can chewMORE THAN something OR somebody ELSEto try to do more than you are able to do → biteExamples from the Corpusbite off more than you can chew• Many kids who leave home to live alone find they have bitten off more than they can chew.