Ok, I have two suggestions that you could try. Both are aimed at reducing the high alert.
Firstly, you need to artificially induce some feelings of calmness. You probably already know that we are wired for threat, and react far quicker to that than we do to signals that we are safe? Your brain probably is now on high alert, watching out for another threat like the fall, and thinks it's so likely, it won't let you rest easily.
Do you have any go-to strategy/ies for calming anxiety? Like meditation, hypnosis, etc? If so, really bash away at them overtime now and for the next wee while. You will need to send many, many calm signals before your brain's alert system stands down. I personally find hypnosis is way quicker and more effective than meditation, but it's horses for courses. You might find affirmations like 'I am safe, I am calm, I am protected' helpful, if you can really induce a state of belief/feeling that way when you say them (hypnosis would help with that).
In addition, when your mind starts to go down the path of 'what if', firmly counteract it with thoughts of 'I'm fine', 'it's ok', I'm healthy', etc. Try and stop any catastrophising, but by countering it, rather than trying to suppress it.
Try also not to beat yourself up about the accident. Shit happens! You will learn from it.
It's an awful position to be in, but hopefully you will be feeling better soon.