I have been doing TKD for almost 4 years and I still cant get my left leg to perform as well as my right. Its strange. My legs do everything together unlike hands where you use your right more than left. (some of us are lfet handed so the opposite is true). Your left leg is probably better at keeping you balanced so your right leg can kick more. I say. do 2X more kicks with the left leg than the right when training. in a fight (sparring or whatever) use your good leg, but give the weak one extra attention outside the ring. have you tried weight lifting on just the left leg? that might help too. unless your under 18 I dont recomend you lift weights yet.