Why can't I hit the tennis ball properly?

I'm 14 and I've been playing tennis for about a year or 2, and I was top on my school one year (not that it's a great accomplishment). I used to use the eastern grip but recently I changed to semi-western. The problem when I used the eastern grip was I hit it long 1 out of 3 shots. Now I use the semi-western grip but I have new problems. I rarely hit the ball deep with good spin and speed and I frequently hit the net or hit pop flies that somehow have a lot of topspin. When I started using semi-western grip, I started twisting my wrist kinda back so the racket wouldn't be at such an acute angle to the ground. But I frequently hit up high and into the net. I usually don't hit it long though. Another thing that might be a factor is I'm using a 100sq in. racket when i usually use oversize. I've been told my strokes look great, but what else am I doing wrong? Any help will be greatly appreciated. Thanks.