1998 Chevy Cavalier runs good until AC is turned on

The car runs good until I try to use the ac. Then the car struggles to go. It has trouble changing gears and it feels like it jumps back and forth when moving. If I stop at a light while the ac is on or park the car it tries to die. If I turn the ac off it runs just fine.
i had someone look at it a week ago and he changed the fuel filter but it didnt fix the problem. Also my gas mileage is 100 miles to a tank less than what I was getting. Please help!