1st get your hearing checked. It is often done for free at churches and some clinics. You may be speaking so loud because your own hearing is somewhat impaired. Join a drama class and use your voice there. Most people have to struggle to project properly. It sounds (no pun intended) as if you do that naturally. That would give you a guide to gauge your voice range and help you realize to tone it down in other places. Also have your trusted friends help you out by gently reminding you. They don't have to say anything. The girls could simply put their hand on your arm and squeeze gently. They guys could elbow you. But they probably won't do it gently! ;P Whisper to your pet for practice talking quietly. Use shower or bath time as an opportunity to let it rip with song. Again, so you are differentiating between what is 'normal' and when you can cut loose.