Coady - I think your issue is that you are afraid of the ball and are hanging back anticipating the ball to hit you.
You need to work on your technique (there are loads of threads on here about that - please use the search function) and you need to work on your confidence. This will come - as a youngster I am guessing that you face boys who are much bigger than you so this will pretty much be the hardest time for you while you catch up in size and confidence.
Have lots of practice.
As a young player I would not advise trying to bring in a trigger movement as you need good coaching to get it right - stand still weight evenly balanced through your feet and stay side on through the backward defensive shot.
As you get stronger you can build in the pull back in.
As mentioned I don't really like trigger movements, but I especially don't get the advantages of the back and across movement. I realise many people use them to great success. I think there are better ways or better trigger movements you can use to fast bowling.
Ponting was as good a pull shot player as any in history and played it with a front foot movement not a back and across