Most people think of dopamine as the "pleasure chemical," but in reality it is the "anticipation chemical." Dopamine says: "You got this. Almost there!" And it is up-regulated when an uncertain profit presents itself, implying that all you need is a little bit more focus and practice.
Too little dopamine and you get OCD and hoarding: the inability to decide. You can think of a hoarder's room full of stuff as decisions left unmade. When you can't decide, you open & close your car door 45 times until it sounds "just right".
And when you have too much dopamine, you do stuff with zero anxiety, but you don't stop to consider the best course of action because you pick the first, best path. Useful when running from a predator, but not when solving complex problems. Just look at people on cocaine to see how this pans out.
The best personal advice I have to handle the ups & downs of ADD/ADHD condition is to build supportive todo & reminder systems around the bipolar highs and lows. Exploit the manic highs and outsource as much rote work as you can. Find ways to help you remember and reward life-sustaining tasks during the lows.
I've been working on some software that helps me get the most out of life in this manner, which would also potentially help older, mentally-compromised patients. If this is something you are interested in, please do reach out.