I completely understand your frustration, for I too, am naturally soft spoken. When I was in eighth grade I lost out on a lead role because I thought projection was shouting (which for me still wasn't very loud!) and no one told me any differently.
In fact I was told to "just shout in my face" by the person I had to talk to in my first CT production.
Shouting will damage your cords over time.
Do you have trouble projecting when singing? I ask this because the support comes from the same place. You need to breathe as if you are singing. I hope this makes sense. I never had anyone explain it to me properly, I just noticed that when I learned to breathe properly, for singing, my speaking projection improved naturally.
Practice it at home, try having a conversation with someone who is in a different part of the house, or across the street. Eventually you won't even have to think about it, it will just happen.