As a 28 year old who was just diagnosed, I can tell you from lived experience that it's the condition, and by extension, the expectations that people have of you that you aren't capable of meeting.

When I was a kid, I constantly described myself as "lazy" and "tired". Now I understand I was really experiencing "executive disfunction" and "understimulation".

Adding those words to my vocabulary didn't cure my ADHD, but it did relieve some internalized shame and anxiety that were caused by living undiagnosed (and therefore untreated).