The thing that fixes not remembering what you read: write something after every paper, immediately, no exceptions.
Not highlights. Highlighting feels like work and produces nothing you can use later - that is the trap, because it leaves you with a marked-up PDF and no memory of it, which is exactly your situation.
A short structured note per paper, in your own words:
- What question it answers, one sentence
- What they did, two sentences
- What they found
- Why it matters to my project specifically
- What I doubt about it
That last one is what your advisor is actually asking for when they ask what the literature says. They do not want a summary; they want your position on it.
Five minutes per paper, and the payoff is that your literature review is then a matter of arranging notes you already have rather than re-reading two hundred PDFs in month eighteen.
One more thing worth doing now: go back to the nine you have read and write the note. You will find you remember less than you expect, which is unpleasant and useful: it will convince you to do this for the rest, which nothing else will.
Keep the notes in one searchable place, and keep it plain. The tool matters far less than the habit.