One of the toughest things to make a habit of in a match is attacking the middle. (That's the midpoint between backhand and forehand, roughly the playing elbow.) There are two primary reasons for this. First, it's a smaller and moving target than the corners, which are easier to attack.
But there's a bigger reason - players don't practice attacking the middle. How often have you practiced your forehand or backhand, going crosscourt or down the line? When you play a match, guess what? You will tend to do what you did in practice, and so you'll go crosscourt or down the line. If you want to learn to attack the middle in a match, then you have to practice it.
So instead of always doing corner-to-corner drills, have your partner block from where his middle would be, with either his forehand or his backhand. Develop the habit of going to that spot by practicing going to that spot.
You should also do other drills where you attack the middle, such as serve and loop to the middle. But it starts with repetitive drills where you make it a habit to attack the middle, so that when you play a match . . . it'll be a habit to attack the middle.