I was already in my 50s when diagnosed with autism. It was mindblowing and overwhelming as it explained so many of the struggles of my whole life. More recently, I've found I also have ADHD, which has explained a few remaining issues which autism didn't.
Having lived the first 50+ years not knowing this, in many ways it doesn't matter now. There's nothing I can do now about the problems I had growing up - at home, school, university, work, with relationships, with church, socially ...
I muddled my way through these things, and most people would think I was 'successful', whatever that means. But it all could have been so much better if I'd known what I know now!
So there's a certain amount of regret that throughout my whole life I've been misunderstood; I've continually tried (and often failed) to do the right thing; I've continuously tried (and often failed) to fit in; and I've been bewildered by things that others seemed to just know or could do. All of these have led to being mentally, physically, spiritually and emotionally overwhelmed and exhausted. To know now that there's nothing wrong with me - it's just that I think differently to the majority - is both a comfort and frustration. It's a comfort because my life suddenly makes sense, and it's frustrating because of the irreparable damage caused by 50 years of being misunderstood and consequently mistreated.
A bigger regret is the effect that not knowing has had on my children. Because I didn't know my brain was wired differently from the majority, I assumed that when my children struggled with the same things I'd always struggled with, that it was normal. Only recently have they each discovered their own diagnoses. I hope that at least knowing in their 20s, they will be able to navigate the rest of their lives better than I have mine!