ATPL theory is not too difficult, not as much as some people say it is. If you are doing the Integrated course, it'll be quicker than the Modular. When starting subjects like Human Performance and Air Law, there is not much to them at all...in fact it's nothing but a memory game. But when you hit General Navigation and Meteorology, they can take a few days to settle in to and get focused on what's happening. What ever you do, do not panic. You will ALWAYS get your head around the topics, especially with a degree in Physics!
I'm just coming up to finishing my second of three ATPL Theory modules, all the tough subjects in this one But certainly doable! Can't wait to start flying