It takes practice, deliberate mindset, and occasionally caffeine or other legal stimulants. Sometimes the books are just poorly written.

First, you need to work on restructuring your mind. Our society pushes new things all the time, you can never have downtime; you must always be on Tiktok, Youtube, Insta, FB, Reddit, Lemmy, whatever.

Starting now, try to be mindful of your time on social media and video platforms. Allow yourself to sit in silence and let your mind race through everything that's being pushed back by the content you're consuming. After a while (a month or two) of this "Social Media Mindfulness", you should start feeling better and able to concentrate on reading. It's not an immediate fix, but it's the healthiest one.

Source/Further information

I would also recommend How To ADHD for tips and tricks, on how other people deal with their ADHD and study habits.