1996 Chevrolet Cavalier Live Northern Minnesota Antifreeze Radiator Car Last

I live in northern Minnesota and have to use antifreeze in the radiator of my car and for the last half year or so the antifreeze keeps disappearing ( though not on the ground ) so apparently somewhere into the engine. I have to keep adding a 50/50 water and antifreeze mix to the reservoir. Any suggestions as to where my antifreeze is disappearing? I know my car has high mileage and it has been a great car, but with the high mileage am i looking at an engine overhaul or what? Thanks much, Dale