Exhaust problems

Ive been having exhaust problems with my 2.7 Taco. The check engine light came on reading Catalyst Efficienty Low Bank 1. The gasket in front of the Cat was leaking so I replaced it and cleared the code. I drove it for about 30 miles and the light came on again so I replaced the rear gasket. A few days later the cat started rattling and when I took the cat off later that week I found that it was hollowed and the pieces must have gone into the muffler. THe cat was new a few months ago right before i bought the truck. I have noticed that the gas milage is a little lower than normal. My question is what would cause the cat to go after a few months? Im planing on a header, high flow cat, and muffler but havent decided what to go with yet. I just want to make sure of the problem before i put a new cat in again and ruin that one as well.