The situation you are in is very difficult. And making a decision is all the more difficult with the emotional upheaval you are probably feeling. I think you are on the right track in wanting to talk to someone in HR about your options. However, I still don't think mentioning ADHD is going to get you any special consideration, in fact, it will likely do the opposite. (Are you hanging on to that to excuse yourself from admitting your skill deficits? Please don't be insulted by this comment. It is natural to look for blame anywhere but with ourselves. I am tempted by that bait, too. I could blame a touch of OCD -- never diagnosed except by myself -- for my difficulty in organizing and managing time and tasks in nursing. But in reality, my best "excuse" is that I'm a new nurse! And it's hard for everyone! Not just you, not just me. Not due to ADHD or OCD, but to the challenging nature of the job even for the most talented among us! And sometimes is really is a bad fit, or you weren't in the right place mentally or physically to meet the challenge, etc, etc.)
There -- now I've said it, but you may decide that doesn't fit you at all. Only you can be the judge.
A couple of thoughts about resigning or being fired. You might go to your local unemployment office for some advice on that. Resigning will cause you to be ineligible for any unemployment, I think. That might be an issue. If you are offered another position in the company and refuse, that could affect eligibility also. On the other hand, if your secretary job had decent pay and working conditions, it could be valuable in keeping your head above water financially while you seek another nursing job. All things to consider.
I can't blame you if you feel returning to your old position, even temporarily, would be a demoralizing step down. I do think that we have to advocate for ourselves and ask for what we want. If you want your orientation extended, ask. If you want another nursing job in the organization, ask. The worst that can happen is they will say "no." But no one is going to hand you a job in the work world these days. As others have said, there are too many unemployed waiting in line to step into your shoes in a heartbeat.
Well, don't know if any of this helps. Take your time, get some good counsel and advice. And go for what you want!