First of all great sympathy from me, and empathy, because I do know what it feels like, my bad fall was 2 1/2 years ago, and I went from being nervous about cantering, to terrified
First thing I had was a very patient and understanding coach, who understood when to push, and when to hug! I also had a horse that I trusted, I knew he wouldn't buck or bolt through the transition, but his natural laziness meant that we had to go through a rough trot before picking up the lope...not fun.
So, what helps:
Visualisation, you were a rider, you know what a canter feels like, I spent a lot of time sat on a gym ball in my house, just remembering what a good canter felt like, and more importantly how to ask for it.
Singing! I know sounds strange, but singing does a lot of things, makes sure you are breathing, relaxes you, gives you something else to think about, and in my case I swear frightened Gibbs into picking up a lope. Mine was "One Man Went to Mow" has to be something you know well and can belt out.
Sit up...when we are nervous we instinctively try and curl up, so we lean forward hunch shoulders, and become ineffective. So sit back a little, head up, practice rolling shoulders back and down, opening up and lifting the chest .
EFT, not for everyone but boy did it help me, Emotional Freedom Technique (EFT) - Emotional Health
I was introduced by a physiotherapist, who realised quickly that my recovery to riding was both mental and physical. Came up with a positive affirmation to say that got me through, it was very helpful.
Bachs Rescue Remedy Rescue Remedy Drops and Spray - Original Rescue Remedy
this stuff is excellent, have used it for years in many a stressful situation, and it does help you relax just a little.