I can only pump a little gas into my tank at a time, and only to half tank. I have not been in any accidents and checked underneath for dents - nothing appears to be unusual. Any ideas?
When you try to push in several gallons of gas into the tank, the same amount of air has to come out. In the old days, gas tanks used to have a vent and most of the air went into the atmosphere, contributing to pollution. Modern cars have sealed tanks and most of the air gets pushed through a system of hoses and canisters (evaporative emissions system) that collect the gas vapors and later burn them in the engine. Some of the air gets pushed out the filler neck and this is what trips the pump handle to shut off.
In your car, too much air is being pushed out the filler neck and shutting the pump off prematurely, because the normal system is blocked or not working.
The most common reason for failure is over-filling the gas tank, which clogs the charcoal canister with liquid gas (it is supposed to only trap vapors).
Other reasons include failures of various valves, tanks and hoses. It should not be too difficult for a competent mechanic to diagnose.
Is your check engine light on?
StrongDreams did an excellent job with an explanation, but it might help greatly if we knew the year, make, and model of your vehicle. Perhaps one of us will be aware of a specific problem common to your car.