In another thread with a similar question I wrote down an execise for getting better at high kicks:
For kicks, do kicks. First have a GOOD warm up to prevent injury. Kick at a heavy bag or for this occasion maybe even better a BOB. Kick relaxed without force and start at a level at which you are really comfortable with (might even be at a low kick level). Let your body accustom to the motion. Do like 25 kicks on each side, do another 25 on each side slow (meaning SLOW), then go 2" higher, same procedure. Go as high as you feel comfortable, you shouldn't feel pain. Don't force yourself to go higher, as there is risk of injury which would draw you back.
After that, do PNF-type stretching for the splits. Keep your legs warm afterwards, either by movement or outer heat (bath, sauna)
Do that every day (start always comfortable low to prevent strain trauma) and you should be able to deliver head kicks after a month.
I wasn't sure about the other guy's technique, but usually technique is a main part of the problem with high kicks. With proper technique you don't even have to be close to be able to do the splits for head kicks. But it's hard to tell without seeing your technique. So just some basics: When kicking pivot 180° on the ball of your foot so your heel points towards your target and your toes away from it. Open your hips (it's a bit hard to explain and your trainer should show you). Bring your upper body down. To give you an idea on how much difference body alignment/ proper technique makes try this: stand upright with your feet together, then go down to touch the ground with your hands 2-3 feet in front of your feet, then lift on of your legs (with your groin pointing to the ground - both hands touching the ground). Feel the limit - and now open your hip (so your groin is at 90° to the ground - one hand touching the ground the other pointing to the sky) and feel how much higher you can now lift your leg.
So besides streching pay a lot attention to your technique and ask your trainer to help you on this issue.