Could be a learning disability, like I said. Could be that math is just harder for you to grasp, or that you haven't been taught to think about it the right way yet. Like I said, we don't have a particularly good track record for teaching math in this country. I couldn't do much math in my head until I was an adult, I was successful at math -- in advanced classes and everything, but I had no fundamental grasp of a lot of the concepts (due to missing most of 7th and 8th grades). It wasn't until I was in college taking stats, but also taking classes on how to teach more basic math, that the concepts really clicked and all of the sudden I was good at mental math -- it was all about how I was taught to look at it.
I like teaching, so if you want, I can try to help you figure it out.
give me a sec.