I feel similar - being used to the distraction of switching between multiple tabs while waiting for a page to load, while a video plays on the second monitor, scanning the feeds on my phone on breaks. I can't focus to read a book and a movie takes days to watch. But it's not the internet.
I used the internet for years without any attention-span decrease or distraction features. When it was gopher, telnet, IRC, email, and BBSes etc. running on a DOS PC with no multitasking. (OK, I cheated, I had a TSR that let me switch between up to 3 programs, but RAM was limited and you were only using one at a time.)
It's the combination of multitasking OSes, putting a lot more things on the screen(s), and ubiquitous pocket supercomputers. That's led us to a new standard way of living where we're always trying to do several things at once and expecting context switches every few seconds. That makes it hard to do anything deeply and fully, or focus on anything for a lengthy time; it feels unnatural.