My left leg use to suck too man, I've seen girls kicking harder than me. Here's what you should do. Once you have the proper technique down go to the gym on an your off day (lets say u train mon wed fri go on tue and thu or somethin) just kick the bag. But only use your left leg. My left leg kicks have increased dramatically in 2 weeks. I can't emphasize that enough. One of my instructors did this and there's a ton of right handed guys who done this and now there left leg is actually more powerful. Of course if you spent so much time on your right it would be powerful too but it's great nonetheless.
ok so it looks like repetition is key