Somebody famous has said (maybe john von neumann?) : "it is not so much that you get to understand hard concepts, you just get used to them by working with them often".
My problem with QM always has been that I was put off by its very weirdness, and I never learned to handle the concepts well in previous attempts to learn the subject.
But thanks to prof Zwiebach's Edx courses 8.04 and 8.05 (and hopefully soon 8.06) I can now well live with them, just as I learned to live with gravity's action at a distance at secondary school.
Indeed these worldclass courses are crystal clear, not least thanks to using all the mathematics needed for it from the start. The courses have a very high level of "learnability" , with all the interesting opportunities to work with the concepts and with helpful, always timely guidance from staff and TA's.