**Moved the thread to put it in the appropriate forum.**
Looks like you have to work on your flexibility. Make sure that you encorporate a lot of stretching (especially dynamic stretching) to your work outs. Kick stretches will help with the range of movement, that all comes with a bit of repetition and time. I don't subscribe to the "super-flexibility" stretch exercises, and I don't know how being able to pull the splits while being suspended between 2 chairs helps you fight. Sure, it's great for book covers, but successfully winning fights can get you there just as quick.
But since you noted that you have a hard time kicking above your mid-section, you might take that as an opportunity to work on your form. As it is, the greatest amount of power that can be generated in a kick is generally from the mid-section down, and high kicks tend to be extremely high-risk in actual fights.
Then again, so are flying armbars. But knowing that you can pull it off is always a great thing. In any case, this book right here is a great start for training kicks. It is written by grandmaster Hee Il Cho, who has been studying and practicing Tae Kwan Do for the majority of his life. Sure, TKD is popular in the MMA ring, but you can't deny that TKD practitioners know how to throw kicks. I have the book myself, and it not only breaks down the kinesthetics of each individual kick, but it also has drilling routines, stretching routines and strength exercises for each kick.
Start working you're way up while kicking the bag, gradually make note of where you're kicks are. I'm about you're size and for the most part head kicks are about complete extension with the legs, but if flexibility is the problem then I'll wait for trainer to reply.
To know for sure i would have to assess u personally, but generally the muscles that are an issue with kicks are the hamstrings (back of thigh) and adductors (groin and inner thigh) also possibly calves.
The basic Idea behind PNF is to stretch the muscle as far as it will go, then Isometrically contract the muscle against a resistance (for 5-15 secs)
then stretch it further, hold the stretch (for 10-15 secs) then repeat the process 3-5 times.
this works much better with a partner, (especially one who knows wat they're doing) but it can be done by yourself
I too have struggled a long time with kick height, im 111kg and 1.82m, so im not genetically built for kicks to the head, leg and midsection kicks are easy and the same genetics make me deliver them with deadly power. I can kick my own height in the head, but I lose all that power and im not balanced at all making me available to damage myself. Its also more difficult to set up and land than ,say a leg kick. Damm that Crocop, he makes everyone want to do what he makes look easy, its not. A good highkick is always good, dont make it too important-its not