Why the rebuild is harder than the diagnosis
The diagnosis itself is information. You meet criteria for ADHD; here are the diagnostic codes; here are medication options. For most adults the diagnostic conversation is relatively brief and produces a sense of relief — finally, an explanation that fits.
The rebuild is the work of integrating that information into how you understand your own past, present, and future. It involves revisiting decisions you've spent decades framing as 'I should have tried harder' and reframing them as 'I was operating with an attention-regulation difference I didn't know I had.' Both stories can be true — character and neurology aren't mutually exclusive — but the relative weighting shifts dramatically. Your understanding of your past performance, your relationships with parents and partners who interpreted your behavior through the wrong lens, your sense of what you're capable of going forward — all of these need rewriting.
The reason this takes longer than the diagnosis: identity is built from thousands of moments interpreted through a particular lens, and rewriting requires touching each moment one at a time. Therapists familiar with late-diagnosis populations describe it as similar in shape to grief work, though the underlying loss is different (loss of the self-story rather than loss of a person). It's not pathological; it's just slow.