I'm only waiting for a chance to be diagnosed with ADHD myself, but I understand this one's generally common among people who get diagnosed later in life (I'm late 30s): For me it would be the loss of time and loss of opportunity to live up to my full potential in life.

Sorry if that sounds depressing. I'm hoping that getting diagnosed and treated will allow me to turn things around, but it's still hard to accept not being able to turn back the clock.