Hip flexor and adductor issues are common challenges in Taekwon Do due to the dynamic nature of what we do. However this risk of injury is exacerbated with students that try to over extend themselves too soon, getting ambition and ability mixed up, as well as instructors who haven't been keeping up with their conditioning and training on a regular basis and then start to over extend themselves from their current de-conditioned position.
Getting over this issue is a holistic one that requires the use of a number of rehab techniques such as static, isometric, dynamic and ballistic stretching and mobilisation techniques as well as general muscle skeletal conditioning exercises, re-educaton of the kicking biomechanics will probably also need addressing (these biomechanics could be the reason you have the condition in the first place!). Once your condition has gone chronic, it will probably be necessary to seek out someone who understands the issues, has the skill sets and knowledge and can specifically assist you and rehab you back to health.
The below video will give you an excellent insight in to weather you are at risk, educate you on the problem and if you already have the issue, how to start to overcome it.