You can focus on the fact that your issues stemmed from what was done TO you- PTSD isn't a deal breaker (I have it, and have known a lot of nurses with it)... They're (BON) just wants to know you're safe. I had to write a similar letter ages ago, and it was sent, reviewed, and end of story
Focus on how you can do your job, you're following MD and therapy requirements, and have been working on getting back to work- you can empathize with patients while remaining professional. Don't lie or minimize things - but focus on what you can do, and not how bad things got- unless it gives you more 'weight' as far as being an effective nurse
Also look into the SSD back to work program, where you have a 9 month trial period, so if for some reason you find it's too much, you don't have to apply for benefits again- you're still in the system, AND receiving some benefits....