Voice Specialist. It can be many different reasons. One common reason is silent reflux. Just had a patient with that. The important point to make is be seen by an voice specialist. You need a laryngoscopy to ensure that you dont have any abnormal anatomy (polyps, hematoma, nodules). Once this is done, you can be directed to the right therapy. An experienced speech pathologist or an ENT will be a good start.
Speech therapy. There are specially trained speech pathologists that can help you. Another source is a good vocal coach to make sure that you have the mechanics of singing properly. Do you smoke? If so, that is hurting your lung capacity which will diminish your ability to project your voice. (plus smoking is really stupid! and kills people).