1998 Mazda 626 Cannot fill gas tank up

The problem is that I cannot turn the gas on and let the tank fill up. I need to put gas in about 1/3 of a gallon at a time. So I have to fill until the pump stops (as if my tank was full), fill, stop, etc. If I try to put in more gas than 1/3 gallon some will spit back out at me. The more the tank fills, the less I can put in at a time. The shop said it would be $300 to pull the tank down to find the problem. Any ideas?