I can get good grades, but I don't understand concepts. Can anyone help me? Do I have a learning..?

I have always made excellent grades, but never understood the big picture of things. I learn alot slower than others, because I don't understand alot of things. I am in anatomy and physiology, and with the paperwork I am wonderful. However when it comes to the lab, I am really bad. I don't understand how to go through the lab work and it's humiliating. At work, I am a customer service representative, and I try really hard to work on my comprehension. I am on adderall for attention deficit, and it helps me with getting focused, and the umption that I know what I have to do. However, when they ask me about billing, or some problem that requires alot of understanding, I am really slow. However my memory is fantastic. They are saying that I am not using my tools and resouces "effectively". my problem is, I can't. I don't understand alot. They are not happy with my performance. I am so upset that I can't learn like everyone else. I am horrible at math also. I want to tell my doctor, but there is probably no cure for this. It's going to cause me to lose my job. Everyone else can do it, except me. What can I do?