I have had the same problem for years, until I got new eyes (cataract surgery). Now I can see everything. But having trouble focusing was why I went to auto focus cameras, and I've never looked back. Particularly, in the studio, you'll usually be shooting a f8 or f11 which gives you quite good DOF. But if you have trouble with the critical auto-focus, then have the model hold a contrasting item right in front of her face and focus on that.
It would be nice if our digital SLRs would show us the actual preview, but alas they don't yet. I have, in the past, set up a small video camera right beside the SLR and in the same plane so that I could preview the shot (I used this in the film days mostly). The other way I overcame focus problems was with the RB67. With a 2 1/4 x 2 3/4 viewfinder it was very easy to focus and compose.