2002 Pontiac Grand Prix Need help!

OK, so heres what was wrong, My car was running reall bad and could not figure out what was wrong, after doing an oil chamge, cleaning the air filter, changing the plugs and the wires, and chngeing the fuel filter we decided to bring it into a shop. The shop told me that my "cat" needed to be cut out and replaced. And at the time I could not afford that, so I just had them cut it out and put a pipe through. I was told that because my o2 sensor was after my "cat" I would get really bad gas millage. So after A month or so I was fialally able to afford to get a cat instaled, it was installed and my check engine light is still on and the error that is still reading is engine missfire. And my car is still getting bad gas mileage. Can someone please help me