I'd wager this right there is the problem.
Sounds like you're reading through books like they are a chore. Instead, find one book with a subject/theme that interests you (so the subject itself is not boring) and then try to relax and read slowly.
If it's fiction, savor every word and phrase, try to visualise the description/emotion/action. If it's non-fiction, think through what what you just read means, why is that's so (it's okay to not have answers, questions are important).
I know "try to relax" is unhelpful. But one approach I'd try is to carve out a block of time (say, 1hr), put your phone/devices on Do Not Disturb, set an alarm for one hour (so you don't think too much time's slipping away) and just read.
It's purely anecdotal, but both me and a couple of friends commented on how consuming short content (eg. youtube videos, short blog posts) led to "attention atrophy". If the content doesn't "get to the point" immediately, we get anxious and bored.
If this describes you as well, try consuming less short-form and more long-form content (longer videos, longer in-depth articles, etc).