You have to ease yourself into it again (well, you don't /have to/, but it takes some action from your side ;)). Fanfic has the advantage that you know the characters and the world building, so it takes less effort to get invested in those stories, than when reading ~original~ ones. Plus, there's many one-shots, or clear chapter divisions that are posted with intervals, so you it's not as daunting to read as say, an 800-page novel (though adding up those 80k+ fic, you do end up there eventually).

I recently started reading more books again, as opposed to mostly fanfic, which I managed through first starting with re-reading old favourites, and then sticking to a certain topic, and branching out from there. That said, you do have to be in the right head space for it. I've had some quite stressful years, and in that period, I simply couldn't concentrate on books which required me to get to know new characters, so I stuck with what was familiar, and that was in this case fanfic.

Try to find some books you think might be fun (not everything has to be high literature, and you don't have to like what the majority does), and don't hesitate to not finish those if you don't like them for whatever reason, or just put them aside for a while and start something else. You could set goals for yourself like reading 1 book chapter for every fanfic chapter you read, or something like that, to maybe have some incentive to start getting into reading books again.