Hi Elizabeth,
I will try to offer something that has worked for me, it may be out of the ordinary but we all find different ways to focus individually.
I too started to lose my balance in balancing poses and was getting a little distraught; so I started to practice a little at home and outside and found that I had pretty good balance, but I was losing it in the studio. I don't know if it was the mirror, multiple bodies, increased pressure on myself or another influence affecting my balance; but what I figured out was, when I'm in class I have to relax(blur) my vision so I don't focus to hard at the mirror and it helps me isolate outside distractions, relaxes me inside and keeps me from staring to hard at myself. After doing this for a year now, I catch myself staring too hard, thinking too hard or forcing my poses at times and I lose my balance, but when I slightly blur my vision and look straight ahead, I feel solid, balanced and focused. I've also found that it.. seems.. to relax my breathing and the muscles to aid in the balance.
I wish I could give you a more sound or scientific cure, but I thought I would throw out what has worked for me. As an avid baseball fan, and Bull Durham being one of my favorite movies, a quote..."Breathe through your eyes"...
I know you will find what works best for you. Relax, work hard and enjoy it
John