I have to say that, in my opinion, you do not have to be whiz in all science course. You do have to have a fundamental knowledge of the body and how it works though. But if you're struggling in a science class it can be more than just "you not getting it". Your instructor plays a role in this as well.
What makes a good nurse?
~Well developed critical thinking
~Excellent communication skills
Nurses are the healers of mind, body and spirit. You can have someone who aces chemistry, a&p, micro, etc.. and just has awful organization and critical thinking skills. So, I think being a great nurse goes WAY beyond science alone! yes, it helps to know that, but I can tell you that I'd rather be taken care of by someone who is personable and caring (as well as knowledgable) rather than someone who aced chemistry and all their science classes but has absolutely no compassion for the ill. That's for sure!
Just keep trying! I am horrible at chemistry but really like A&P. I think along the way, you find your "nitch" and go with it!