Catalytic converter issues.

Okay so I have tried reading and searching the forum for help with this. I have a 2010 Aveo and at a loss to what to do. Started having the car hesitating and losing efficiency. Finally threw a misfire in 2 code. I replaced all the spark plugs and the boot on number 2 and that seemed to help for a couple weeks. Needless to say issue came back and then I got the P0420 code. I got some BG44 K thinking it could be fuel injectors. That seemed to help for a week then the P0420 came back. So like a fool I got the cata6converter and had it installed. I drove off and the issue was still there. The next day I took it to Chevy and of course it drove fine there. They just looked at it and they say that it didn't mess up on them. They say they checked the O2sensors and everything was in spec and that it is the cat still and I need to replace the one I just had installed. So is this my course of action or what. I need this resolved ASAP so my wife can have a reliable car for my 9 month old son.