Any suggestions on how to train my body to constantly stay low?

I often stay low for the first few minutes of a match, but as soon as the pressure is put on me, i tend to lift my head, this is my greatest downfall as i often miss out on goal scoring opportunities and lose the ball

More reps of close-in skills. More reps, more reps, more reps. With reps comes muscle memory, which will prevail for longer when you start to fatigue or come under pressure. It also builds confidence, so pressure is less "pressure" but normal playing conditions.

You may also benefit from strength training that focuses on your leg strength (squats, lunges, speed skating-style leaps, etc.) so that you fatigue less quickly.

Buy a shorter stick. Use it at training. You'll have to bend more to use it properly.

Not a smart idea. Your hand positioning would be completely different on a shorter stick.

You just have to keep training and keep doing drills that require you to stay low, as keely has mentioned.

concentrate on always having your stick in the control area in front of you. If your stick is on the ground then you have already saved yourself 1-2secs of reaction time getting it to the ball.

Edit: You also must concentrate on correct posture: knees bent and back straight

squats and lunges! all the time! it helps get your muscles stronger, other training is just playing around with the ball and focusing on staying low.

Core work is important to keeping low and will sustain your performance through a game. www.hardcorehockey.co.uk has some great core exercises.