I don't want to be the one to explain theology however, if you have recognized that there is a possibility of God but still don't believe it, you are an atheist and not agnostic. To be agnostic means that you never looked in to the subject and don't really care to find out whether there is or not. Essentially, you never made any effort to think about whether to believe in God or not. Hopefully that clears your questioning. I am neither of those, I am a fully faithful Catholic however it can be said that being Athiest is better than being agnostic because at least you took time to recognize whether to believe in God or not. By the way I fully respect your choice, and I won't shun you like other (extremely faithful) Catholics may.