Do you have allergies? Rashes when you eat some food? Trembling hands? Some skin problems? Hearing loss or tinnitus? Are you abnormally tired after sports? Do you have abnormal cramps after sports or drinking alcohol, or when not sleeping enough? Regular teeth health issues? Do you have a sensitive vague nerve? Did you faint more than other people in the course of your life? Does your body need more time to heal? Do you feel dizziness when standing up? Have you always had the impression that you actually aren't that stable on your feet, even while actually practising a lot of sports? What about headaches and migraines? When you work, have you specialised in some tasks that are/feel quite systematic for you? Are social interactions or other tasks that require new analysis exhausting for you? These are signs that you optimise your energy expenditure by focusing on tasks that barely require you to think with much energy.
Lack of energy is a very good reason for the brain to avoid using energy for its executive function, the function that is actually impaired in people with ADHD. It's normal, and trying to fix that feels exhausting, which is why people with ADHD avoid spending that energy.
The executive function is preliminary to focusing with other parts of the brain. If it's impaired, the brain won't even try to focus with other parts for the actual work. Two problems solved at once. Twice the energy spared.
Find the root cause of your ADHD (which often is just a symptom) and fix it.
There are also exercises which help develop short term memory like n-back training for which there are many phone apps.
Also doing something mindless helps. The dishes, the laundry, moderate exercise.
I can't pay attention to most audiobooks at 1x, I get bored between words.
I have ADHD myself, and I advise you to always keep reading. Don't stop and think about what things meant, don't recall the important events at the end of a paragraph or chapter, none of that. Just keep reading.
If you really feel like you didn't 'get it', go back 3 chapters. But reread it without stopping.
Or, you could try the opposite, and slow down. It sounds like torture, but eventually, it will become habitual.
Just like a movie, if I really like it, I could decide to read it again, and discover things I didn't get the first time around. But there is no requirement to spot every detail the first time.