I honestly think the first semester is the hardest. Not necessarily d/t the material you have to learn but learning how to adjust to nursing school. Nursing school isn't like any other classes you've taken.
You need to learn organizational skills, time management, how to study (if its been a while since you've been in school), what these new teachers expect, how they test etc. Clinicals are scarey when you don't know what to expect etc. Fear of the unknown is the part that makes the 1st semester the hardest. Oh, and learning critical thinking can be the end all/be all for some.
My classes were different from greatan's where they learned the diesease processes first, but we learned what was the normal way the body and all its components work, then we learned what happens when it doesn't. I think that would be easier to learn the norm first. How can you tell what is a normal lung sound when you only know abnormal? If you know what it "should" sound like, then when it sounds different than that you know something is odd.
I've talked with mod 5 students in my school (we have 5 mods, each mod is comparable to a semester) and they said that the first was the worst, and then the last is a close second. During in the ones inbetween, things all seem to come together. You are building on things you've learned and you don't even realize your way of thinking has changed from the first mod/semester.
Best of luck in school! Its a long and bumpy road but the end is so well worth it. (I graduate in 7 months and 20 days..but who's counting