As Nasser said, the best way to gain confidence is to practise a lot.
You need to trust your horse, they have 4 feet and most of them have great balance. Make sure you don't bother him being to forward, keeps your shoulders back, stay half-seat but close to your saddle and let your horse do the work ;)
Simpley practice practice practice! Start with no jumps , then simple jumps then proper jumps and so on. It's actually easier to cross country than to show jump coz u don't have to worry about strides and leads