Seems like you're getting your high and lows, and ups and downs mixed up. Low gear is biggest cog. You shift down to get to the lowest gear. High gear is smallest cog, you shift up to get the the highest gear. The low limit screw prevents the derailleur from moving past the largest cog and into the spokes. The high limit screw prevents the derailleur from moving past the smallest cog and into the frame/dropout.