Please help! Self conciousness / anxiety when performing and singing! Help?

I've had a lot of issues with low self esteem for quite some time, and
I've finally started having counselling. My counsellor has told me to
take any significance away from appearances and just to focus on the
things I'm good at, to make me feel good about myself. One of the
things I'm good at is singing. I practice a lot, and I'm very good
when singing alone. But as soon as I have to sing infront of my
singing teacher or colleagues, I freeze and get tense and sound
terrible. Like REALLY bad. It's making me feel terrible, because the
one thing that was making me feel good about myself no longer works.

How can I get over this? What if I never do? I'm terrified that I'm
going to make a fool of myself and I cannot relax!