After years of struggle and failed attempts at fixing the problem, you evaluate evidence and notice it fits the "I have ADHD" hypothesis more than it fits the alternate explanations.

Happens more often with the Inattentive subtype of ADHD than with Hyperactive, because the former is much harder to spot, especially in adults, plus everyone around you is eager to offer you value judgements as diagnoses (suggesting you're lazy, undisciplined, don't have an actual problem), and/or prescribe you the common "go away" nonsolutions like better diet and exercise (I call them "go away" because it's just sending someone on a wild goose chase in order to have them stop bothering you).

ADHD is severely underdiagnosed in adults, and getting from undiagnosed to diagnosed stage is often just a matter of going to a psychiatrist - after first overcoming stigma, stereotype, and ADHD-induced problems. Easy peasy. Not.