FYI, I haven't viewed it yet but, according to the Melbourne cinemas, this is Pt1.
From all I've gleaned from the trailers & IMDB it sums that there would be another movie-or two-to finish this project.
If I'm reading it right then this 'first' issuance is predominantly focused on the "Loser's" habituations rather than the constant flash-backs one experiences in the novel.
This is too bold, but, (what a weird and, at times, industrious word) in hindsight-perhaps-allowing the story to unfold in 'present time' then working back to the recipient, and experiential, knowledge of their youth with an ending of the first film in a cliffhanger of "what next" segueing into the chronological prequel mayhaps have more merit.