Im the opposite, im a English rider born and bred and am now doing western. The one thing I will say is that it is all about posture. There are some different riding movements like posting but like everybody always says and you will always be told... 'imagine there a string from your sholder to you feet'. You have to sit straight and then you will have to get used to the reins. Also your feet are in different positions... my biggest thing to overcome when I started western was the way I was trying to move the horse. I was using mouth insted of neck rein.
One suggestion that may help you... go to the riding critique section and see how some people are sitting etc and how they have their horse etc and see the comments, you will quickly learn this is the way to sit or this is not the way to sit. Etc. It will take time and practise but good luck and most of all have fun. :)
:~: A Horse Is An Angel Without Wings :~: