Partition problems

Hi, newbie here. I tried to make a partition in Disk Utility to install OS X Lion. Currently, I'm running OS X Mountain Lion, but it slows my macbook down like h*ll. Apparently, making a new partition is the only way to reinstall Lion. During the process my battery died and the partitioning failed. So I opened Disk Utility again, but it didn't show the partition I was trying to make. Problem: even though it failed, it took the amount of memory from my startup disk.

How can I get that memory back? HELP?!