Many players are afraid to make a mistake or they are afraid of the ball. To these students I say, "touch the ball." How many times have you started to go for a ball, then a fear of making a mistake overpowers you? In doubles, when you are at the net, force yourself to touch the ball. Nothing happens in tennis until you make contact with the ball. Every ball that you touch is one less ball that can hit you. Plus, once you contact or touch the ball it has a chance to go over the net and stay in the court. Picture the ball coming into your racket, rather than your racket chasing the ball--this concept will also help your serve return.
Have you been in a situation in a match where you are so afraid that you run off the court or you turn around to not get hit? This does not teach your partner to have confidence in you. Do you see the pros do this? No. Next time you get in one of theses situations, stand still, keeping your racket in front of you and just use your racket as an implement to protect you by just touching the ball. At this time, forget about winning the point; just "touch" the ball.
If you are getting aced, try just touching or making contact with the ball. As you get more comfortable making contact then you can start taking a larger swing at the ball. When you are at the net and caught in an aggressive quick exchange, think about just touching the ball. Later, as you become calmer and more confident, you can start thinking about making better shots.
Overcome your fear of the ball by "touching" the ball. You will see that your hand/eye coordination will get better and better. Remember, nothing is more important than the actual contact point or "touching" of the ball. Once you understand this, your game will move to greater heights.
Good luck on the courts!