2003 Hyundai Accent Possible problems and how to fix em?

Engine Performance problem
2003 Hyundai Accent 4 cyl Front Wheel Drive Automatic 100k+ miles

A few years ago my mother bought me my first car. I've been keeping regular maintenance on it such as changing the spark plugs and wires and the engine oil. Although I've done these things I seem to still have alot of problems, but that doesn't surprise me because of all the mileage on it.

Most recently, I've had more problems now than I ever have. I lost my license and moved out of state, leaving my car with my mom. I was gone for about a month and when I returned, I started my car and the first problem I notice was a knocking sound in my engine and the engine light was on. Secondly, it seemed as if the emissions were abnormal. They smelled odd and didn't seem to be coming out of the exhaust pipe the way they should. The smell was something like gas and eggs all put together and a few other things thrown in the mix. Also, it didn't seem to have much power behind it. When I would put the car in reverse it wouldn't move, but I wasn't the shifting. While having the car in neutral it idoled very well. There was no knocking sounds and the smell kinda went away. I drove the car and while driving it seemed to sputter and kinda wanted to give out. Go dead. Shut off.

I consider myself to be somewhat educated on engine and stuff, but I can't seem to find my problem. I've even taken it to an auto store so they could put it on the diagnostics they have and the only problem they could come up with was the spark plugs and wires. I've changed those several time since then and still the same problems occur. Recently I cut the catalytic converter away from the engine and exhaust and it seems to have more power and run, but I don't know what reprocussions that could bring. Although running better, the check engine light is still on and the engine still seems to want to skip.

What's wrong with my car and how should I fix it?