ADHD is like a minute to minute attention thing. All the issues with ADHD involve inhibition and persistence, the ability to stop and start tasks, organize your thoughts, motivate yourself, etc. It is an issue with executive functions and it's neurodevelopmental, meaning you can spot the symptoms before bipolar starts, BUT bipolar can worsen the symptoms (in my experience). Think of it like a car with a brake that's too old.

Bipolar is an episodic disorder. I may be fine for 2 months then suddenly I'm slammed with an uncharacteristically devastating depressive episode. I go from completely happy to mildly miffed to severely disturbed, trying to will myself to stay alive. Then, as fast as it comes, it disappears. Depressive episodes usually last three or more weeks. Hypomania lasts several days. Mania hasn't happened in several years thank God.