Here is a little drill you can do to see if your hand stays behind the ball as it is reaching your slide foot. You will have to kneel for this. Kneel down on the floor with the knee of your balance leg and in front of that put your sliding leg at a 90 degree angle so your sliding foot is in front of your balance leg knee. Adjust until comfortable.
Now put your thumb and fingers into your ball and let the ball hang straight down from your shoulder with your palm facing forward. Now all you have to do is swing the ball back and forth all the time keeping your palm facing forward which will leave your hand behind the ball. As it is swinging forward close to your slide foot it might feel uncomfortable. You have to get use to that because that is what you want. Now here is something to watch out for. Leave your arm relaxed as you are swinging and when it gets close to your slide foot see if your hand turns an inch or two towards you as it goes passed your slide foot. Sometimes this will happen automatically
because your brain is signaling to your hand that staying behind the ball doesn't feel good so it tries to get your hand to turn a little to feel more comfortable. This might sound crazy to some people but it works for me and might be the case for you. The trick is to make sure your are is relaxed because tense muscles in your bowling arm will not let your arm rotate past the slide foot like it should. That one to two inch turn of your hand is all you need for a 45 degree rotation. So do this little drill to get yourself use to stayuing behind the ball until it reaches your slide foot. If you are lucky that little turn of the hand might happen naturally for you and not something you have to think about. Hope this will help you.