First of all, I'd try to borrow an English saddle. You can't really get in an effective two point position in a Western saddle, and your being unbalanced may contribute to the horse scooting off after the fence.
Second, before using ground poles, placement rails or cavalettit, make sure you understand the theory and purpose behind them. It is possible to make a bad situation worse by misjudging the spacing and having the horse step on a rail or trip over cavaletti.
I'm sure if you do some Googling you can find some good explanations and diagrams to help you devise appropriate exercises.