Dealing with Severe Drowsiness: Seeking Advice

I experience severe drowsiness when doing things that require mental effort. I've had to deal with it since I was a child, but it got worse as I became an adult and had drive. In the beginning I would get drowsy every 20 minutes and had to pull off to the side of the road to rest. One time I drove 70 miles mostly asleep, but promised myself to never do that again. It's been several years and I've trained myself to stay alert or stop at a rest stop for breaks. My psychologist recently told me these bouts of drowsiness are not a known symptom of ADHD or Bipolar 2. I may need to seek help from another specialist. Any advice would really help