It's obvious that you should practice your shooting. But there is a difference between shooting when you have all day and no distractions and when there is a man in your face. Often times the defender doesn't even touch the ball but he
1)Forces you to shoot with a different motion
2)Distracts you

You should make sure that you have a good form. Many people shoot with bad form when it's just them and the hoop. But when there is a defender the hands get to low and away and it's an easy block. Ask yourself could I shoot with a man in my face with this motion?

You should also practice with distractions. At first you can just practice with anything that distracts you. Then you should move up to actual practicing with a defender. But he doesn't need to defend all out at first.