Does my 2000 Honda Accord V6 have a bad Cat Converter or a bad Oxygen Sensor?

I recently bought a 2000 Honda Accord V6 4 months ago. It has been driving beautifully. A few weeks ago I was driving the car and it began to shake really badly and I was able to get home. The next day I got in the car and started to drive and for a few miles the car would seem to lose acceleration. For example I would push on the gas and the car would go, and then the RPMs would drop even though my foot was still on the gas and then suddenly start back driving. After a few miles it would start driving normally. The check engine light came on and it said that the upstream oxygen sensor was bad (bank 1, sensor 1). We had the oxygen sensor replaced and the car drove fine for a while. Now it is back doing the same thing and the check engine light came back on and is saying that the same oxygen sensor is bad, and we just replaced it. What could it be? Is it the cat converter? Also, some other things I'm noticing is bad gas mileage, a burning smell while driving. And some other strong smell coming from the exhaust.

There are about 6 different things to check when that code comes up. Google that code and it will tell you.

A mechanic know a ck engine light is a sign, NOT THE ANSWER. For example, Naybe you car has over 100,000 miles and the sparkplus are misfiring, sending raw fuel into exhaust system.hurting the o2 sensor, Its called DIAGNOSIS.
!buy a shop manual 30 dollars ant parts store. You will instantlu become smarter
2. Find a good mechanic, Car is 15year old, many thing need service at that age

Check to see if the sensors in the car has a heating element and see if that heating element is nornally functioning. If it is good, looks like your cat has a depleted catalyst and needs to be replaced.

I had a similar issue on a 92 Dodge Spirit I used to have back in the day. It turns out that there was a small amount of engine coolant seeping through the head gasket into the combustion chambers and getting passed through the exhaust. Engine coolant contaminates oxygen sensors and renders them bad after a short time. You will probably want to take your car in and have it tested for combustion gases in the engine coolant. If those are present, then you will most likely need a head job and new head gaskets.