Maybe read some murder mysteries with very tightly woven clues. You can't know which details are important to the solution and which aren't, so you need to read it all.

You'll probably try to skip anyway, but halfway through you'll realize you missed something, and then you have to go back and read a part again, fully.

Or you could read very short short stories. If there are only a few hundred words, there's not enough for you to skip anything. Once you're used to reading a very short story in full, you can move on to slightly longer stories, carrying the habit over.

Orrrrr you could try to read webnovels that post weekly. There's only one chapter at a time. It's ridiculous to skip anything, especially because the end doesn't exist yet.

Orrrrr you could read a book out loud, maybe to another person or just to yourself. If you're reading out loud, it's harder to skip anything.

Ultimately, it's just retraining your mind.

Oh! You could try comics. Less text, more pictures. Kind of hard to skip pictures and speechbubbles.