Two words: chair seat! You need to get your but more towards the front of the saddle and get your leg back under your but. You will be sooooo much more in balance once you do this! You will be amazed at how much more stability you have in your position. Probably getting you in a saddle that suits you better will help.
Once you do that you can learn how to get your horse more forward, and then to take a more positive contact on the reins.