Breathing during front crawl

Question
I was a complete beginner and have been learning to swim for 4 months.  I have been practising front crawl and can turn my head to alternate sides every 3 strokes,take a breath and then blow it out when i am under water.  The problem is that when i take a breath, i do not seem to get the air into my lungs and then have to stop after less then 1/2 a length to get my breath.  I have tried everything and i just can't get any further.  I have slowed my strokes down and do rotate my body and my leg kick is good.  My instructor cannot offer anymore advice.  I would aslo consider myself as very fit.

Answer
Linda--
It sounds to me like you are actually gulping your air, taking a shallow breath, not breathing into your lungs.  The breath should be just like your head isn't in the water.  Try holding on to the edge of the pool and practice your breathing technique, deep breath in and blow into the water.  You said you have on been swimming for 4 months, give you self some time.  Being physically fit on land doesn't necessarily equal fit in the pool, the endurance will come with time.
I hope this helps some,
George