Quantum Mechanics? - Where should I start?

I've finished third year but not sat the second semester exams yet due to a mental health issue. I sat with my Quantum Physics by Stephen Gasiorowicz textbook but I don't understand the problems presented. I looked at the solutions online
https://www.slideshare.net/kadubrito5/gasiorowicz-quantum-physics-3rd-ed-solutions-2 but still don't know how they got there. There's gaps in my knowledge and I only got a C for my second year module of Relativity and Quantum Mechanics. Should I brush up on the second year stuff first or dive right into the third year stuff? I got straight A's for my third year tutorial sheets but really don't know what do to with the past paper questions... What do I do? I've not gone through any textbook problems yet. Are they a waste of time? Should I just continously search for/ask the lecturer for past paper help instead of doing textbook qs? I still haven't figured out the study method for university yet.