Like Grace said, you have to build up it's confidence so that it starts to like jumping more. The easier, the more fun! And it's the same for us humans, right?
So start slow and really try to focus on the confidence and don't forget to have fun! <3
Straightness, adding poles guiding the horse to the center of the jump can help. If height is intimidating to the horse, just set up a grid and build the jump height into the grid. Also, just start low, ask the horse to jump it, and slowly build up.