Practice and overtime it will feel natural!
When it comes to riding a big horse, start easy and work your way up with the difficulty of the riding. With the corners really use your inside leg to push him into it and make sure you stay upright and don't lean forward. This will help him stay balanced.
Maybe do figure 8s around barrles and keep moving them closer together over time? It's works for me, but it could be something else that works for you. Just trying to help.