(Note: My son has diagnosed ADHD, non-hyperactive) There isn't a blood test, but you can do a neuropsychological evaluation which assesses cognitive functions. We had one done for my son, and it was a 2-day affair. As a non-ADHD parent, it was eye-opening to get a glimpse into how his brain worked and to start to really understand the challenges ADHD poses in a school environment.
As a first step, you might hire a coach to help you with
1. Making and keeping the necessary appointments.
2. Dealing with insurance.
3. Persuading your parents to endure the ego-threat of listening to your actual needs instead of telling you things like "stop being so hard on yourself".