I had this same problem for quite a while. I had force my mind to accept that I probably didn't pick up everything the first time around. I didn't let that stop me in my tracks. I simply went onto the next thing. Eventually, something would come up, usually a compiler error, that would force me to flip back to some section I have previously read. I would find out what was causing the error and fix it. I have stopped trying to learn it all, and instead I read once and then reread only what history has pointed out I failed to pick up on the first round.