I think watching the khan academy series on heart physiology might help with the understanding behind diastolic and systolic blood pressure if theory is what you are having issues with. Perhaps being able to visualize what is occurring internally will help you with the actual skill of knowing what your listening for. Otherwise just practice more, on yourself, your roommates, your pets, hmm maybe not your pets but you get the idea.
As for strength I wholeheartedly agree with the above poster! Join your schools gym and start lifting, as a new lifter you should find gains that will make your work as a CNA a lot less worrisome. It might be intimidating at first, but most gyms have tours where staff will show you the basic safety precautions and what not.