It has happened to me as well as to people I've known, though it's been some time since I did any jumping!
It helps to just go back to a smaller height, and make some lower jumps of different geometries, and then go back to the height later.
I think that you're at a point where a lot of people tend to get tense. I think it's kind of a mental barrier, getting into more complex obstacles, a little higher etc.
I think it's important to just give yourself time and not put a ton of pressure on yourself. I think your horse won't be tense if you aren't tense.
Or...I COULD do one of those Milton Erikson things....he felt people responded to language in amazing ways, so he'd say, 'These things don't get solved OVERNIGHT, they don't go away INSTANTLY, or in THREE DAYS, you can't expect these PROBLEMS to just DISAPPEAR'...and the person would usually go back and their fear would have disappeared, 'like magic'....LOL.