Breathe. I had one bad accident during a jumping course and I was scared of starting again. I used to picture the accident every time I entered the arena. You just have to go from the basics to the complex. I started with poles at a trot than poles at a canter, little single jumps, higher single jumps than poles and a jump...You got to boost your confidence from the bottom to the top. Also, ride a horse you're 100% confident with.
If possible. Ride a horse that you know takes amazing care of their rider, and doesn't ever rush. (Like a well versed hunter or a really good school horse). This is in no way required, but may give an extra little confidence boost.
Then start with single cross rails on the quarter line, coming in at the trot. Slowly build it up to a gymnastic (trot poles to cross rail, then a one or two stride to another cross rail. Gymnastics are great for building confidence because they usually don't have fillers, there's little space for a horse to run out, and you don't have to worry about finding your distances.
You can also work with pole courses at the trot or canter, followed by trot poles. Always remember, it's ok to trot a fence, or circle to gather yourself.
One you get to the point where you can ride courses again, get someone on the ground to keep reminding you to breathe, this will help you relax. Also, just count your strides, all the time. The constant rhythm will give you something to focus on, and will help your rides to the jump.