I had the same problem and its likely because your attention is used to jumping around, thinking of something else while doing anything. Its really common to be like that now. I notice it when reading books the most. I had to make big changes like not listening to anything or looking at my phone while working, reducing multitasking. Limiting my utube watching to 1 hr total on the weekend. Deleting social media apps from my phone and having to re-download to check once a week. Deleting reddit. For me its not one solution just while I'm studying. Its a gradual process of training my brain to calm down and focus. Locking in is incredibly hard if you have scattered your attention for years. I wish someone told me that earlier.