How do I avoid whacking the ball

Hi there sirs,

Unlike most players, who seem want to hit hard, my problem is that I have developed a terrible habit of mindlessly whacking the ball. I mindlessly try to hit everything hard without thinking or control and although this is alright against players of a lower level, it is not alright against players who can keep the ball low and/or spinny. This is the first time I have encountered this problem and no idea why; before when I hit hard I always place the ball, put spin on it and control it but now I lost all of that. Is this normal and how do I regain my old style?