I'm glad you went to A&E due to your self harm, that's a very brave thing to do! - How did you feel about taking the step and dragging yourself over the Accident and Emergency? :3
It's difficult to say thank you to specific nurses, but you can say thank you through a website they make, they often have a specific E-mail or Feedback page, often complaints are feedback, so similar I would assume, are you in the UK? NHS? Maybe?