Not staying back on pitches is the result of these 3 primary things.
1. Fear of getting beat by a fastball.
2. Not being able to read the spin on the ball out of the pitchers hand.
3. Being Nervous. Based on another post you made I am going to guess you are nervous at the plate and over anxious.
The one suggestion I havent seen offered to you in this thread is instead of just addressing the mechanical aspect. You should also address the mental aspect. Until you can learn to control the mental aspect of this problem no drill in the world is going to help you.
You obviously feel the next to attack the ball out front of the plate so in order to help correct this you need to learn to trust your hands. This drill will help with developing that muscle memory and tach you and show you that you can allow the ball to get deep in to the plate on off speed pitches.
Have your coach throw front soft toss to you without your bat. Assuming you are a right handed hitter. Setup in your stance and set your hands up as if you were holding a bat. Now using ONLY your top hand as the ball comes across the plate catch it with your top hand. Use the same trigger, load and stride that you would if you were taking BP.
Test yourself and see how deep you can let that ball get before you unload. Have your coach soft toss inside, outside and down the middle. Relax and just push yourself to see how deep you can allow the ball to get.
Also another couple of drills you can use just incase you are not picking the spin up on the ball out of the pitchers hand.
1. When someone is throwing BP to you in the cages. Have them where a black batting glove on their throwing hand. It will allow you to see the ball better coming out of their hand. This will help you focus more on the ball.
2. Typically when a pitcher throws a four seamed fastball you should see more white in the ball as its coming towards the plate. When a pitchers throws a two seamed fastball you will typically see more red in the baseball as its coming to the plate.
So here is the drill. Have your coach or whoever is throwing BP change their grips up between two seam and four seam. After they pitch the ball and you hit it tell them whether it was two or four seam. This also will help you to focus on the ball coming out of the pitchers hand and help teach you to recognize pitches. This in turn will also help to teach your brain to get those signals to your body faster.
Hope these help and best of luck.