Not me but DH who was diagnosed in his 60s within the last year.
He was in denial until he had the official diagnosis and even after that quite resistant to any suggestions I made for strategies that could help him. It was putting a strain in our relationship. In fact his condition has always put a strain on us but now we know why and are working out what helps and what doesn't.
Like you, he missed opportunities, educationally and career wise because ADHD wasn't identified at all until the 1990s. His self-esteem certainly suffered.
Fortunately he found a career which matches his hyperfocus and did very well at it.
I help him by pointing out the hard evidence of stuff he's really brilliant at, that everyone lives a life with the cards they are dealt and all the positives he has.
He says things are easier now as he has more understanding of how his brain works.
There are some very good YouTube videos and podcasts which might help you (search ADHD on Mumsnet to find the recommendations).
If it's not too woo, could you also try some short daily gratitude practice? Search Spotify and if you find it difficult to sit still and do it, try listening with headphones when you're walking.
And give yourself time to grieve, that's fine, but make a conscious choice whether that should consume you.
Good luck.