Sure, nursing school is hard, but it really isn't all that difficult either. Seriously the hardest part of nursing is learning to think
like a nurse. Doing A&P is difficult in and of itself but remember that they're going through systems and how they work. Over time you'll start to see that the systems and the underlying physiology is interconnected so that something that has an effect on one system often affects other systems.
If you're having trouble grasping a concept in A&P, I suggest you get with someone that understands the system and can teach it to you. Then here's what makes things solid: teach it to someone (or something) else. I'm sure that there are thousands, if not millions, of educated cats, dogs, rats, and chickens around but since they're who they are, they just can't.
The major benefit of the teaching was that the teacher
was able to put together coherent thoughts about A&P and verbalize it in such a way that it all made sense.
The same thing happens with nursing too. You'll be presented with parts of the whole and, at first, it just doesn't seem to fit together quite right. That's because it's missing some things that will be filled in later. As you progress through the program, those missing pieces will be provided to you and as long as you recognize that you were just given it, the whole begins to take shape. It's not going to be until near the end that everything pretty much fits together and you can see the whole, even with missing bits. Your "lightbulb" suddenly gets lit and things just make sense. Same happens with A&P. It just takes time for all those pieces to be put together.
If you're getting grades sufficient to get you into nursing school, chances are pretty good that you'll be able to get through nursing school. I pretty much guarantee that just about every program around tracks student performance and matches that up with program completion rates and NCLEX pass rates and they adjust their prerequisite grades/scores accordingly. Yes, it' does mean they've got a lagging indicator, but over sufficient time, they develop a very good understanding of what is necessary for a student to succeed.