As one who was "let go" at the end of orientation, I know how devastating this can be and sympathize wholeheartedly with those in this situation. Trust me, though it may seem like it right now, this is not the worst thing in the world that can happen. It can be a blessing in disguise. I truly think that if I had stayed on in that job, I'd have spent many a miserable month, with high stress, continued anxiety, struggling to prove myself, overwork and poor self-esteem. I'd have felt dissatisfied on many levels, not least of all with the lack of communication and constructive feedback. They were right. It was was a poor fit in many respects.
My advice, for what it's worth. #1, DO NOT bring up the ADHD. At this point it might sound like an excuse, and might be additional cause for employer's concern about your fitness for the job. The stigma exists and could bring more complicating factors into the equation. #2, As others said, try to get some specific evaluations of your problem areas. (This I did not do, yet, but intend to do so soon. You will need this to explain to future employers how you are remediating your deficits, and for your own use to improve your skills and knowledge. Also to find out if the employer will provide a job reference.) #3, You must list this job on applications, though not necessarily on resumes. I list it on both because I believe the experience was valuable regardless of the fact I was not ready for the acuity. #4, Be very careful how you explain the resignation or termination, whichever it turns out to be. You can easily give a prospective employer reason to throw your resume in the "do not hire, ever" pile if you don't choose your words carefully. Wrong impressions are hard to overcome. You may not have another chance. #5, get some counseling, talk to trusted friends and people who respect you, keep checking in to this forum (there are many stories like your and mine), do some journaling around all the feelings of shame and disappointment, take care of your health, and get out and have some fun!
Being an older worker myself, the hurdles and obstacles can seem overwhelming. I have had to work very hard to maintain my goal of becoming a useful and successful nurse, and overcoming this huge blow to my ego (also not entirely a bad thing
) . I believe I will eventually find my niche, where I will feel valued and respected, and enjoy what I am doing. You will, too. Good luck and do not give up.