This may be splitting hairs, but I'm not sure I'd put "assertiveness" on too many lists of must-have characteristics for a good manager. A manager can't be a pushover - i.e. people need to know they've got to do what they're told to - but a newly-promoted manager who starts acting too "assertive" is likely to end up with trouble.
It sounds like thats not going to be a problem for you. Don't worry, I've known a lot of "shy" people who were great managers. Look at some of the skills on the list I linked to, and see which of them you might want to work on.