The 2nd phase of Yahari Ore no Seishun Love Come wa Machigatteiru. analysis, focused on "Yukinoshita Haruno's wish to help her sister, Yukino" after the firework festival. Also the true meaning of her remark, when she's talking with Hikigaya Hachiman & Yuigahama Yui.
The first phase before Yukinoshita Haruno decides to help her sister, Yukino. Maybe these clues might be the starting point for you to understand Yukino's problem(s). Such as, how Yukino feels about the car accident.
If we know who is the owner of the third memorandum, so many detail that you might never know about this person will appear. If so, just whose monologue was it?