Sometimes those assessments can go wrong. People with high IQs often don't get diagnosed, and self-reporting symptoms can be tricky because you're being asked to describe 'abnormal' traits / experiences when, if you have ADHD, they seem very normal and unremarkable; for this reason, it takes a great deal of self awareness to be able to notice and articulate symptoms. Even neuropsychological examinations have limited utility in diagnosing ADHD (ask me how I know haha). That said, there could be many things going on - poor sleep quality due to sleep apnea or narcolepsy (often co-morbid with ADHD also), mini seizures, etc. It's probably not a bad idea to speak to a doctor again. Unfortunately, if it is ADHD, it's often hard to get a diagnosis in adulthood due to medical professionals having a poor understanding of the condition, or thinking that anyone exploring an ADHD diagnosis is pill seeking, so before you speak to anyone it's worth really taking stock of your life, talking to close friends and family, and trying to learn what, if any, behaviors / experiences felt perfectly normal to you, but are not shared by others. For example, if you're asked 'do you have trouble sitting still for long periods of time when expected to,' you may be inclined to answer no, because for you, shaking your leg and playing with objects might feel like a perfectly normal feature of sitting still, but others who have spent time around you might answer the question 'lol yes' on your behalf.

If it is ADHD, even if you do get a diagnosis from a psychologist, be prepared for a battle to have it treated with medication. Many psychiatrists won't even consider treating you without a full-on neuropsychological evaluation (despite their limited utility), so you may need to be ready for the longest, most expensive (in the US anyway) standardized test of your life, and still possibly needing to fight your case.

In the interrim, I think fidget toys are a good idea; for me, I'd choose something like blue tack to make shapes with, or maybe doodling. Recording the lectures is probably also a good idea, so that you can go back and repeat sections you've zoned out through. Personally, I find stimulant mediation is more useful for executive function than sustained attention. (Of course, if there are also sleep issues going on, it'll naturally help you there, but you really ought to be taking the stuff under medical supervision so best not to experiment on your own) I also haven't been to school in years, so I never cracked this problem myself, I'm afraid.