I had a similar situation the first time I joined Peace Corps. When I wasn't doing great at school, my parents sent me to a psychiatrist who said I had ADHD, prescribed me...something...which had absolutely no effect. My issues were not ADHD, but rather just I wasn't enjoying school (at the time). This was about 8 years before I was invited, so I had no idea how to get in touch with the psychiatrist I saw, and I was worried that if I just wrote a statement saying I didn't have ADHD in the first place, that they would just laugh when they saw it.
So, I ended up (on PC advice) having to go see a random psychologist, discuss my psychological history and current mind set, and he wrote a statement stating that he believed the diagnosis was in error, and that I was having no current issues.
That seemed to work.