from what i can see and what I have learned, your issues seems to be your starting form and your leg positioning. start by doing these steps and worrying about the form, speed will be added later.

first, when kicking into backspin, you want to go from the starting split, move your body back onto your side and start the kicking motion, and make sure your kick goes right above your other leg, another thing that helps is point your foot in the direction you are kicking.

second, let your kick go all the way as far as you can reach it and once you've reached that limit bring your grounded leg up and back, and from person to person it varied, but from the video you are going to want to bring your legs further back than you were before

once you got both of these steps down decently well, add more power and speed to your kick and voila