The string-orchestra concert that I worried so over has come and gone, and for the first time ever, I was happy with the way we sounded (onstage, anyway), and felt good about my performance. As we reached the end of the Ives, I could not stop smiling - we'd played my favorite piece in the world and not made a mess of it, and I could just sit there in the middle of us and listen, and enjoy it. My husband said he'd never seen me smile playing viola before.
One thing that helped immensely was that my cellist friend and I got together a lot outside of formal rehearsals, and worked on stuff, some picky, some not - counting (especially in the Ives), holding notes for precise lengths of time, getting our intonation just right with each other, changing the feel just a little bit this way or that, trying to match vibrato, etc. It was a lot of stuff that we knew would get drowned out by the rest of the orchestra, but my friend was sitting principal for the first time, and she took it very seriously (even though I was principal of my section too, I knew it was only because Mr Serious University Music Student didn't show up until partway through the semester, and he ignored me anyhow, so I did not feel the same responsibility to my "section" that my friend did). We worked on the music as though we were the only ones playing, and the net result was we knew those pieces backward and forward, and were able to pick it right back up after some mishap (which did happen) and keep putting whatever heart and soul we had into it.