Scientifically speaking what is the best way to teach myself how to focus and study after 6 years of complete discipline erosion(being an undergrad and then a NEET)?

I find it hard to keep my focus on something that's not explicitly entertainment for more than a few minutes at a time.

I just spend the entire day reading books(fiction) and masturbating and I'd like to fix that but I find it very difficult if not impossible to go through a textbook or online courses.