I'm probably going to hide this after I post because ND and NT-partner-of-ND support should not mix, but I will just leave it here in case it is helpful (but won't see any replies).
I have ADHD and in my experience, the diagnosis has a process similar to this:
I had been reading about ADHD and learning stuff for months leading up to the diagnosis, but it never felt quite legitimate, I had this idea "Oh, I'll just try the ADHD strategies and see if they help!" but I could not realistically do that without the validation of the diagnosis because I kept flip flopping back into a kind of imposter syndrome, feeling like a fraud, feeling I wasn't "entitled" to whatever it was.
Then the diagnosis. Huge catharsis for me. Allowed me to go all in on the strategies where I somehow had not before. I don't really know why. For me it was a 7 year gap between diagnosis and starting proper medication, which is unusual (long story).
When you've been undiagnosed with an ND condition right into adulthood, you have developed all kinds of coping strategies and masking, most of which are actually stressful and/or maladaptive. In order to create new, functional coping strategies, the old, maladaptive ones need to be unbuilt and unlearned. I've highlighted it because it seems like sometimes there's a perception that people should just be able to instantly swap from one coping strategy to another but it doesn't work like that. In the meantime, while the old strategies are being disassembled, things get worse, because now you have no coping strategy. Everyone is an individual, and ASD will be different to ADHD, but honestly, although I had some improvements immediately to my own mood, I would say it probably took 3-4 years before I got out of the kind of "building site stage" and actually had solid foundations to build my new coping strategies on. During this I thought that my DH might leave me because I felt at several points that it would NEVER get better and maybe this was the best I could ever be, probably because I didn't directly recognise that I was going through a process of unlearning old, unhelpful coping mechanisms or that I was then struggling through without any mechanisms. When waiting for assessment it can feel like after I get that answer everything will magically be easy and it is not. Nobody really offers you a plan of what to do next. However, now I have got out of that kind of "building site" I can see improvements and it really does make a difference. I don't feel like I'm a different person to who I was initially, though I have no idea truly what that is like for someone outside of me. I'm sure that I went through some over-defensiveness during the "demolition phase". I know that I linked EVERYTHING back to ADHD which frustrated DH. It was just something that I needed to go through to get here which is a much better place than I was previously. The obsession was actually helpful, and some wallowing was necessary processing.
Anyway, that is my (one person's) experience of the process from the other side, in case it's helpful to anyone.