I use to have that problem as a vocalist. I didn't know about it until it was brought up. I still have that problem in public. My Mom and others have told me I look like "a deer in the headlight''.
There's really not much you can do about it. I guess you could try looking in a mirror and work on softening your eyes. I remember being in voice lessons and my teacher wanting me to look in the mirror to watch my mouth. Maybe the same thing could work for your eyes?