I too am typically reserved. I had my first therapy session yesterday and on my way home I kept scolding myself for not bringing up one topic or another and stuff that I honestly didn't think of.
So when I got home I immediately started to write them down. Each and every thing, no matter how significant or insignificant, it's in writing. That helps me in two ways - first, I wrote it down, it's out in the open (even though it's private) I've written it, I've read it, and not it's real. Second, I intend to bring the list to my next session and I'm going to read it. How do I know I'm not going to chicken out? I'm going to tell the therapist I wrote the list down she'll ask me for it.
BTW - I'm at TWO PAGES of narrow margins of things I want to say. Just writing them down makes me feel better too.