Things you might want to try:
1) standing desk
2) varying the time of day in which you read
3) stimulants (from coffee to adhd meds)
4) find discussion groups of something intellectually interesting in person (or uh places like here i guess)
5) change your diet, in particular try cutting out/reducing sugars
6) cut out other superstimuli like social media (or at least any with likes), vidya, porn
7) less ambitious works, scaffold up with wikipedia, etc
8) meditation
9) look into sleep hygeine if your sleep is off (bed only for sleep and sex, get up/go to bed at consistent times)
IMO if you throw these against the wall one or two is likely to be helpful, even if it doesn't solve your problem entirely.