Palmtree:
Don't feel bad. If you are extremely nervous about the O/W part you are right to say "I'm not ready for this, I need some more practice."
I had a bit of trouble with the mask clearing too, though my problem is that every time I THOUGHT I breathed out through my nose, I was actually breathing out of my mouth and through the regulator. No matter how hard I tried, I kept breathing out of my mouth rather than through my nose. It was hard to overcome as I finally got myself used to breathing in/out through a regulator, now I had to learn to breathe in via the mouth and out through the nose (the reverse of what I learned to do under stress). Finally, another experienced diver who was on the same boat with me told me a trick: when you need to clear your mask, pretend you are sneezing. That finally did the trick for me.
I am a bit surprised that your dive shop won't do a private lesson. They will likely charge more for it, but I've never heard of someone turning it down for the right price.
You've gotten good advice here, perhaps spending more time in the pool and waiting until the next O/W session may be the right thing for you. Or, if you want a private lesson but the shop still refuses to do one, perhaps you should ask for a "referral" form and do your O/W sessions with another instructor, maybe even in a resort-type environment away from home.