Izzy is right about the last science thing. I applied for the spring 2010 ACE program only to find out that there was no chance of being accepted because I was taking 2 of the 5 science courses (A&P II and Microbiology) and needed the 'points' from these courses to make my application competitive. The woman was really nice and actually sat down with me and we went over my score sheet. I have a 3.53 pre-req GPA and scored 93 (99th percentile) on the NET when I applied.
I would definitely continue on with the chemistry courses. As Izzy noted, you'll likely be required to take them if you want to advance your degree at some point. Additionally, you'll need the basic chemistry knowledge for A&P, any nursing entrance exam you have to take, and I imagine it will help you out in pharmacology, too. Even if they are difficult for you, I think it might be good practice in perseverance for when you actually start nursing school. Nursing classes won't be a cake-walk either and there are no 'substitute' courses.
Find study groups, ask questions after class, read the material prior to lectures, record the lectures and listen to them again while reviewing your notes. Even if you feel you aren't doing well at this point, you can turn it around. Best of luck to you!