I'm with you. Believe me. Fortunately I have perfected the art of looking in a mirror without actually seeing myself. I see you and I see movement but that's it. I focus on the ballet not the bodies.
Which is why it sometimes makes it hard to dance bigger. If we are feeling self-conscious for some reason (our hair looks bad, we forgot makeup, the shirt clings too much), we make ourselves physically smaller. We pull our arms in, often by the elbows, and our centers collapse. Chins droop to the chest, shoulders round forward and pretty soon there are no lines or shapes moving in the space but messy blobs.
It's actually harder to dance when you make yourself inconspicuous!
When you avoid the mirror, you also have a tendency to put your body in the wrong alignment for poses and turns, etc. By not looking at yourself (or that direction), you cut off the correct epaulement. You are not truly croise or efface; you skip ecarte and peer over with your eyes rather than your head. And when you turn, you don't spot because you are afraid to look.
The cumulative effect of dancing inside the box for yourself is that you never truly dance. You never fully move and express yourself. The lovely lines we work so hard at the barre are lost when you come to center and that's a shame. The center is the time to put all of that work to good use! Show it off!
"For whom?" you ask.
Well, me! Show it off to me!
And to yourselves. Don't be afraid to put yourself out there in a combination. You may not have a goal of performing on stage but that doesn't mean you can't perform in class.
My TL:DR advice?
1. Stand up straight; pull up through your spine and all the way to the top of your head and bottom of your feet.
2. When arms are en haut, make sure they are truly over your head and not merely in front of your eyes.
3. Look in the mirror at the audience, not yourself. If that's frightening, think of it as "downstage."
4. Take up more room. Don't worry, you won't hit anyone else. Be bold and as tall as you can.
5. Dance fully. When it's your turn, don't mark it or hobble off stage if you make a mistake. Own it.
6. Smile to yourself. Enjoy the music and the movement and know that you're creating art with every step you take. No one else can dance exactly the way you do.