Best advice: relax. Contrary to our instincts you want less tension in your chest, throat and jaw to produce increased volume.
Projection was really my only issue when I started working on my voice so I spent a lot of time on it. My therapist had me spend the first few minutes of our sessions just releasing the tension in my body. The tight, mousey sound is caused by the locked muscles, particularly in your jaw. Think of the stereotype of nervous, anxious people (i.e. me) sounding all quiet and stifled; it's cause of the stress screwing their voice up.
You can do exercises where you hum at correct pitch and with increasing volume. Put your fingers on your lips while you do it and you should be able to feel the difference in resonance when you are doing it right, versus when it's all tight. At first you can chew with your jaw while you do it so you don't lock up, but eventually you'll learn to do it without that and it will integrate with your regular speaking voice.
Note this won't help with screaming. That works differently.
