No one has said it yet but honestly you probably don't need to stretch that much. MT high kicks are based largely on form. I am not very flexible and and I can still head kick when I feel like it because I threw so many high round kicks when I was doing MT that I know the form. Its very likely that you aren't pivoting your support foot enough. when your heel is pointing at your target it opens up your hip and lets you kick higher. When its not you need all sorts of flexibility to overcome your bad form.