Need to be more assertive....

I think I've finally reached a crossroads where my quiet, easy going nature is going to start affecting my career. My review says in a nutshell that I do good work, but I'm too laid back for my position. I need to become more extroverted and every once in a while say what's on my mind instead of taking 20 years to formulate it exactly the way I want it to come out.

I really want to do this because I want to progress in my career. The question is, can I do it without behaving like someone that I don't want to be?

Fiddlesticks...