I thought exactly the same when I was doing MST121. I'd done the TMA's then got them back and wondered what the heck I was talking about. It was a mixture of scary (because it'd be needed later) and frustrating (because of the time it took to get through and seemingly coming out with nothing).
Turns out I was wrong though, and it was sinking in, though it didn't feel like it at the time. For me, the maths usually goes:
1) Read the book, struggle, finally understand, practice, complete the assignment.
2) Get assignment back and realise that I didn't commit anything to memory and get annoyed.
3) Have a quick skim over the book again and notice that actually, most of it did sink in as bits come flooding back. If it took me an hour the first time around, it might take a few minutes and a couple of practice questions for it all to come back.
4) Realise that by doing that, some gaps were filled and what I did remember was reinforced. It's at this point it starts to get stuck long term.
Since realising all of my study seems to follow the above pattern, it's allowed me to relax a bit as I know it's sinking in, even if it doesn't feel like it at the time. What you say certainly sounds very familiar
Edit - there is form of attempted advice buried in there, it's just verging on incoherence due to coffee.