I have a similar problem. In my public speaking class the advice I received from everyone was that I didn't speak loud enough, even when I thought I was speaking loud and clear with confidence. I'm currently reading Set Your Voice Free by Roger Love. It's a fantastic book. I haven't gotten to the exercises yet, but have learned so much already! Here's an interesting fact:
> The human voice is set up to speak or sing twenty-four hours a day without getting hoarse or strained or creating any physical problems. If yours can't, it's because you're doing something wrong. [page 11]
I highly recommend this book! I initially got it from the library, but purchased it from Amazon because the library book didn't have the accompanying CD.
Voice is something that you can totally work on. You can sign up for a signing classes, or voice classes, and voice coaches. There is a ton of resources on the web. I can recommend this book as well: http://www.amazon.co.uk/gp/product/0316441589 (the paper version includes a CD with exercises).
The good news is that if you have a weak voice, it might be very easy to improve with just a few exercises and a few sessions. Squeaky voice would be harder to improve, but still doable.
As for the confidence - if you can afford it, check out counselling / therapy. It can do wonders, and as a 32 year old one of my biggest regrets was not going to a therapy 10 years ago...
Just remember - both with working on your voice, and on your self-esteem, that it's all about finding the right way to work on that. You might have to go through a few books/coaches/exercises before you find the one that fixes you. Too many people try just one and say: "oh, reading/therapy/exercises are not for me".
Three final confidence boosters I'd add - dance (Salsa, Tango, whatnot), yoga, and learning public speaking (see Toastmasters). Those things will teach you to be in control. And if you can move or speak, it doesn't matter how tall you are. (I wouldn't recommend gym - gym may boost your confidence, but might not improve your social skills, whereas dancing and speaking will!)
Is this one any good?
This book should help get you started.
I've used Set Your Voice Free and had a lot of luck with it. I think the real key is just to record yourself often and experiment.