If you don't want to go to a speech therapist, a friend with some linguistics training or a voice (singing) teacher may be able to listen and tell you where to put your tongue, etc.

I, too, have a related problem. I have great difficulty controlling my volume. That is largely hereditary (or nurtured by my family environment), but the real problem is that I can't hear when I'm too loud. There are certain triggers (being excited, interrupted, or in the presence of my mother) but they are not really triggers I can avoid, and I can't see a way to fix it. The obvious solution is to have someone tell me when I'm too loud, but being interrupted for that purpose tends to make me involuntarily louder.