Have been training in taekwondo for about 18 months and am struggling with flexibility, I am 35 years old and think I started this too late, I warm up, stretch and cool off correctly as advised by the class instructor and do this every 2 days for 30-45 minutes (used to be every day but was advised against it in case of injury) again as advised and instructed, my range of motion improved dramatically after about 3 months of this but my flexibility stopped after this spell of progress, I stretch continuously and have no pain, ache or any tears pulls or injury to my legs, hips hamstrings or joints. Its as if my body has "reached its limit" and now no longer has anything more to give. My range of motion is no better than waist height after a full body warm up and stretch and I physically am unable to gain any extra height or rotation with kicks. I am physically fit, have plenty of stamina to carry out regular exercise and can stretch to my limits regularly without discomfort but physically am unable to reach any extra range, my next grading is coming up really fast and I am expected to have an improvement on technique which obviously includes range and distance from waist height and above.
Any advice here would be very much appreciated.
Thanks

I hope Im not too late to help here, but I think I may have an answer for you.

I too was same as you, but after 5 years of taekwondo I could still only reach waist height! Taekwondo is more a martial art that teaches strength, respect, determination, stamina, and control. You have to follow outside/additional sports for flexibility, true speed, and patience etc, which is necessary to really improve. Im not saying taekwondo doesn't teach those aspects but it does not focus on them in a way that other styles do.

How can I explain...
I do certain exercises that I learnt from doing 5years of taekwondo, soccer, and tips I got from family who trained for marathons. If you want to get flexible I can suggest either doing ballet exercises or if that's not to your fancy then taichi or taking gymnastics on the side will help you to improve. However, if you want fast results I can send you a special exercise I came up with. I have not done taekwondo for about 3-4 years now, no training at all and I used to do nearly 6hours of sport/week way back when. I did this 30min exercise program using correct techniques (just these past 5days actually)and after about 3rd/4th day I could already touch/go past touching my toes which on day 1 I couldn't even touch my toes. I am by nature not a very flexible person! Note, a human being's body has no limit, that's why old men of 80-90 can be found doing taichi or the splits etc. (everyone has strengths in different places is a point yes, but there are so many different techniques/styles out there that can help you improve where you want to improve the most and also the martial arts you do must become a way of life for you, that my new taichi master taught me... very important!

Remember in Taekwondo you are taught to think fast, be quick, and simply deliver. A lot of other styles if you research a bit will show that though this is effective it is only good for one purpose and that is defense and attack, it does not really explain in depth how and why and what to do to make each punch more powerful and more effective using less energy. Once you study the background and real know how, the actual underlying bases of how the technique was developed from the start, only then can you really grow and experience the art in its true essence/form.

An example, when you punch the punch bag, do you focus on just punching the punch bag, or have you analysed that by tensing and relaxing certain muscles in your arm you can deliver a more powerful blow, and by swinging your arm further backward you can create a larger force. When you hit the punch bag do you aim to hit just the punch bag or follow through and try to hit 'through' the punch bag, almost as if you are trying to drive your fist right through into the other side.
See just hitting a punch bag will make you fast, it will make you strong but to a point.
If you analysed and change a certain part of the way you punch then only can you increase in speed, in power etc.
Different techniques teach the body different movements and controls.
How is your breathing? Remember with teakwondo breathing properly is essential to how hard the blow will be to your opponent. If your breathing is incorrect then this will slow down your punches, and you will be using more energy than necessary for each punch.

What's the old saying, in order to take out the opponent you need to understand the opponent, well in order to take out the punch bag you need to understand the punch bag, in order to use the full potential of the style you are learning you need to fully understand the actual style from its very roots.

Whatever you do, just don't give up! I believe with your dedication you are well on your way to achieving your goals. The human body and mind are amazing gifts, don't underestimate what they are capable of achieving, the limits one experiences are limits set by oneself, release yourself of these boundaries, believe all is possible, and when you believe then it will come of itself. Remember you are your body's master, so master it, don't let IT master you!. Wishing you the best of luck!