Bite Off More Than You Can Chew

Someone once told me that I had bitten off more than I could chew.  I didn't really see how he could be the judge of that, but I try to be open to criticism, so I spit my gum back out at him.  For some reason, he got all disgusted and stomped away.  I guess he was just so used to receiving a negative reaction to his criticisms that he just didn't know how to respond.