Autodesk Sketchbook has an automirror option, which will mirror it either horizontally or vertically(or both) as you draw, so it is helpful.
If you don't have Sketchbook nor want to buy it, just copy half of the texture that you have already drawn, turn it horizontally and place it on the other side, you may have to do some clonestamping or smudging for it to fit seemlessly but thats the general idea.