Struggling with Quants

The last chapter of QT is a nightmare crying

Agreed. The CFAI book moves quickly enough through the concepts. If you try to learn through the condensed version of the material that’s already condensed, it’s going to be very difficult to pick up. If you haven’t done so already, I would work from the CFAI book, use the notes to summarize and lectures to apply the concepts. You may find that it takes much longer than you were expecting to get through R11, but by R13 you should’ve picked up on the terminology and breeze through.