I used to be the same and it felt like no matter what i did i couldn't figure it out but through trial and error this is what i learned:
-swing like half a second earlier than you think and you'll miss far less often.
-play close to the net (but not too close)
-always just aim for the other side of the court than she is on
-if the ball is going too slow or low back up and let it bounce before hitting it back and it'll speed up most of the time
hopefully that helps!