I used to have that habit. It helped when I jumped to say out loud “eyes up eyes up eyes up” the 3 strides before during and after the jump. Practice practice practice really helps. Good luck 🐴
If you are looking down hold a paper plate in your mouth. I did it as a little kid and I haven’t looked down since. It makes it so that there is nothing interesting for you to look at lol
First you need to know that it is really important not to look at the jump because you will lower your head and your body will follow this direction, you have more risks to disturb your horse or make a fault.
To fix your eyes up, you can fix a precise point behind the obstacle (such as a tree or even put a colorful object that you force yourself to look at). When you're on a show, always look at the next obstacle, it will also help you anticipate the direction to take and your horse will feel it.
My teacher told me something very funny that helped us, you can imagine that there is an handsome boy behind the jump, believe me, you won't look at the obstacle ;)
I hope i could help you :)