Poles, poles, poles!
One of the biggest problems with jumping related fear is that the rider can't see the distance. With that being said practice going over millions of ground poles, for a couple reasons:
1. You won't do damage to the joints, as would jumping over a million fences.
2. Your ride to a pole is ultimately the same ride to a fence, it shouldn't change with an increase in height.
As you ride to your "fence" or "pole" count your three stride. One. Two. Three. On the third stride your horse should be going into take off. If you are having a hard time counting strides (when the lead front hits the ground), just practice counting in rhythm. One. Two. One. Two. Around the entire arena if necessary.
Make sure you also practice two point a lot, even if you are at a halt. I took a clinic with Daniel Stewart, the current head coach of he US Olympic team, and he emphasized doing sport specific exercises off the horse. (This clinic was focused on sports psychology/confidence. Connecting to what I said above, the first day was dedicated to distance work). The most memorable exercise was equestrian jumping jacks, jumping up and landing in two point. This will help you develop muscle memory. Personally, I feel a lot more confident over a fence when I know I have the ability to get into, and maintain a proper two point.
I love what Dapples123 said! When you show 80-90% of your course is actually on the flat! You really shouldn't jump more than once or twice a week!
A good trainer is an absolute must! Make sure that you feel like they are giving you tools to be successful even when you ride by yourself.