To hit the ball a long way you need everything to work together, you need good balance. As soon as you try to hit the ball hard you loose your balance and you don't make solid contact.
Make sure you have a good stace and you are pointing in the right direction, feet , hips and shoulders. The ball position in the correct place.
Make sure you have a full swing and you wrists are fully cocked at the top of the swing.
When you make the down swing keep balanced and don't try to hit the ball so hard that you loose your balance.
one final point to remember don't worry about keeping your head still all the great golfers move their hear sideways ( but not up and down, unless your tiger) . you need to move your weight slightly when you swing as this helps you transfer you weight.
Good luck
martin