Cacti...

Me and cacti do not get along. I don't know why I keep killing every single cactus I get. I can keep african violets, regular ground covers, and other plants just fine. It's just cacti that I have a problem with. I give them water when the soil looks really dry to the point the pot is almost weightless. It's winter, so I try to water them less due to slower evaporation rate. But why. Why do they keep dying? How much water is too much and exactly how much is too little? I can never know...