Why sudden hesitation and really poor gas mileage

Maybe fuel filter

It won't be anything that wears out if the problem started suddenly. Most likely a sensor that failed. -- It's a 2001 car so it has around 150,000 miles. A likely suspect would be a O2 sensor. If there is a CEL, get the codes read at the parts store.

fuel filter, maybe but I'd think ignition or sensors as tenspeed said. Getting the codes read would take alot of guess work out.