I had a similar problem, and I found that there are a few stretches that help with this.
the most effective one I found is to find a ledge about belly height (higher than hips but lower than chest) put one foot up, heel down/straight knee and turn out your supporting leg. at this point you should feel the right muscles stretching, if you need to increase the stretch you can bend your supporting leg. your knee should bend away from the ledge you are using thus increasing the stretch of your groin/thigh. try to hold this stretch for 10-15 sec. then switch legs. I do this stretch AFTER training because the muscles are warm and the stretch is more effective.
another simple stretch is simple sitting on the floor with your feet together and pulled into your groin creating the "butterfly" position. hold your feet with your hands and gently push down with your elbows stretching out your inner thigh/groin. This can be done anytime, watching t.v./in bed/almost anywhere.
don't be too upset, head kicks require alot of flexibility which will come with time.