I was diagnosed ADHD by going through the NHS, it is worth the wait. The forms are extensive and I went for two interviews at hospital.
These people you saw, they sound like they don’t know what they are doing. The reasons they gave you are nonsense. Maybe the best thing is to forget about them but keep on waiting for your NHS assessment. This website is great, although more American-oriented than British. Don’t doubt yourself but keep on looking for ways you can better understand yourself and put things in place to help you with your life and education and relationships. I’m sorry you had a bad experience.