ADHD doesn't always (or even often) manifest as inability to focus; that's a misconception. As well, ADHD
can have little-to-no impact on your grades in school, if you happen to "have a mind for" the particular things you're studying. It's when you
don't that it can be a problem.
Also:
> its effects were almost unnoticeable to me
Neurotypical people don't tend to experience Adderall as "unnoticeable." If your brain chemistry starts off in the reference range (for neurostriatal dopamine reuptake latency period), a dose of Adderall would take you out of the reference range, such that you start exhibiting signs and symptoms of mania, experiencing things such as an elevated heart rate and increased sweating. If a dose of Adderall puts you into the reference range, then you were likely outside of the range to begin with. (Note: if you consume a lot of caffeine, your baseline state would be "you having not had caffeine for a week." If you don't even remember what that's like... well, that's diagnostic for ADHD, too.)
Anyway, give this adult ADHD self-diagnosis questionnaire a read-through: https://add.org/wp-content/uploads/2015/03/adhd-questionnair...
Other possibilities:
1. Despite being considered a different disease, narcolepsy is effectively treated by the same drugs that treat ADHD. Narcolepsy can cause the same symptoms you're talking about (which, again, if you think narcolepsy involves fully falling asleep, that's a misconception.)
2. Alternatively, you might have any of a number of conditions that affect sleep quality, such as obstructive sleep apnea, chronic fatigue syndrome, or even a simple nutrient deficiency. A regular doctor (who would in turn likely refer you to a sleep clinic) could help to figure this out.
3. Of course, it might be early-onset Alzheimer's... but that's actually much less likely than one of the above options.