You are opening your shoulders and chest too much and too early. This is causing the racket to drag behind in the swing. You need to get the racket below the ball and should finish with the shoulders, arm, and racket in a line pointed toward the net with the rackethead up high. Also, you need to get your front knee bent - for many balls the knee should be bent at 90 degrees.
See the clip below(and there are lots of others).