Fuel gauge mitsubishi?

i have a 00 mits eclipse gt auto and fuel gauge wont go down when i fill up the tank completly to full the gauge only goes half way and then car dieds wat could be the problem? i was thinking maybe the fuel pump assembly or fuel sensor cuz when i first got this car the gauge worked fine nd then i put in mix gas like twice nd it uses supreme only maybe i mest up the fuel pump or something could anyone help me out thanks