I'm the same way. Oddly, extreme troubles with math and things involving sequences and direction are very common among people with spina bifida (as is nonverbal learning disorder). They aren't sure why.

I know a guy who's as bad or worse at math than I am. It's more common with girls, but it's not just a gender-programming thing. (God knows my parents tortured me with enough supplementary math tutoring that I wasn't given the message that girls were supposed to be bad at math.)