The technique that unlocked this for me and that nobody suggests: reread.
Take something short and slightly hard: a chapter, an article, a short story - and read it three or four times over a week rather than reading four new things.
First pass: slow, some lookups, mostly getting the shape.
Second: noticeably easier, fewer lookups, you start noticing how sentences are built rather than what they mean.
Third: close to fluent, and this is the one that changes something. You are reading rather than decoding, at your actual level, which is an experience you may not have had yet in this language.
That third-pass feeling is the thing to chase, because it is what reading is supposed to be, and constantly moving on to new harder texts means you never reach it.
It also builds vocabulary better than the same time spent on new material, because repeated encounters in context is exactly how words stick.
On your paperback: keep it. Put it away and try it again in six months after a dozen graded readers and some young adult books. The version of you that opens it then will have a completely different afternoon, and finding that it has become readable is one of the more satisfying moments in learning a language.