I was diagnosed in January of last year, and it's been life changing for me. As @ItsMutinyontheBunty said, I have a much better understanding of myself, my behaviours, and the relief that has brought is huge. My diagnosis removed a lot of the pressure I was putting on myself to be 'normal' and has allowed me to be a lot more accepting of how I am.
I was referred back in early 2019 by a mental health nurse, who was helping me to come to terms with several health conditions that have left me disabled and greatly impacted my life. Like your psychiatrist did with you, he asked me at the end of an appointment whether I'd ever considered that I might be on the spectrum. The wait time for an assessment was already long, but then Covid came along and it took even longer.
I was sent forms out a couple of weeks after he made the referral - one for me to fill in, another for a parent or family member who had known me since childhood to complete. After sending those back, I was offered an online video-link assessment in December of 2020, which was with an OT and a Speech & Language Therapist. Following on from that, it was recommended that I attend a diagnostic appointment. This was with the same two professionals, and it lasted for three hours with breaks. They spoke to my mum on the phone prior to my appointment to gather whatever further information they felt they needed about my childhood. The appt was a mixture of questions and tasks. At the end of the appt, I was asked to stay online and wait whilst they went to discuss their notes with one another. They then came back and told me I have an Autistic Spectrum Condition. (They told me that they prefer to call it a condition as opposed to a disorder, because the wording is more positive).
Other than the obvious relief of having a diagnosis, the aftercare I received was absolutely brilliant. They offered a Post Diagnostic Education Programme (again, online with a small group of other newly diagnosed ASC people). It was a 7 week course, all about living with autism, communication, relationships, sensory processing, etc, and I found it extremely helpful. I was also referred to an Occupational Therapist, who spent several hour-long sessions (again video-link due to covid), and she was able to help me with strategies for managing my sensory issues, how to manage day-to-day issues, and tied them in with my other health conditions.
I got a full written report from both the diagnostic appointment, and from the OT, along with a 'To whom it may concern' letter confirming my ASC diagnosis, and explaining a little about the difficulties that can bring with communcation, interaction, sensory issues, etc.
I realise I've typed a lot here, but I hope that you find some of it helpful. All the best with your assessment when it comes around.