Just an update: I've finally managed to make a start on an original kata. I have to have it ready to perform at my rank testing in mid-July; I think I can make it.
For any of you who are also doing this, here's the bit of advice one of my instructors gave me that finally clicked: start with the pattern of a kata you already know, but use different techniques. Just moving in a familiar direction allowed me to make a start.
The other instructor advice that I've taken to heart (but that didn't help me get started) is "play to your strengths." I'm a good kicker; I can kick high with good form, and I can do good spinning and turning kicks, so there will be a LOT of kicks in this kata. I'm NOT a good jumper, so at most there might be a flying side kick in there; or, if I can improve enough, a turning jump crescent kick or jump spinning reverse crescent kick (but those are unlikely, and my ankle is very sore from stubbornly over-practicing them on Monday.)
Thanks for all the advice, and let me know if you have other ideas.