He felt that he would never again be free of the image of himself as a "mouse"-- a Jew-- this literary device that he had created to convey his parents' predicament as Polish Jews caught in the Holocaust. More than this, he felt that he would never again be free to do work that was not, in some way, connected to MAUS and his fame as the person who created this hybrid tale/tail.
He has said he didn't want to be another Elie Wiesel (writer about the Holocaust, who was actually a Holocaust victim himself). He realized he had nibbled his way into a Maus hole...
The mouse in Maus represent the Jews, when he said he felt like there was a "5000 pound mouse breathing down his neck" he was talking about his guilt, Art always felt guilty for not being through the holocaust, he felt like a hypocrite for writing about something he never went through, but also the fact that so many of his people including his parents suffered through a great event yet here he was alive, not having to struggle and living an easy life.