Tonight my fiction writers' group is meeting to discuss my new middle-grade novel in progress. They're a very generous bunch and have been willing to read the whole thing for this meeting (it's not super long). I feel extremely fortunate to be matched up with such an insightful group of writers and readers who are good at giving useful feedback (I'm not just buttering them before the meting, I swear). For me, being part of a critique group is an essential part of my writing process, whether it's for fiction or playwriting (though they operate very differently).
For my playwrights' group, I rarely get nervous ahead of time, probably because we all bring in work to every meeting, and we bring in shorter segments (10-15 pages) at a time. It doesn't feel like that much is at risk (though sometimes, if I try something way out on the edge, I might worry).
I always get nervous before our fiction group discusses my work, partly because it comes in bigger chunks (this work-in-progress is about 38,000 words), and partly because the work we're discussing is usually the result of months (sometimes many, many months) of labor, and I often wonder if I have any objective perspective left after rewriting fictions over and over again. Internally, I always have a balancing act going on, enjoying the anticipating of sharing work with them that I really like, but also reminding myself to be realistic, that it still needs work, and that I need to make sure I stay open and receptive to comments and the discussion.
For any writer in a critique group, that's the most important challenge--the group can only be useful for you if you're mentally in a state where you're ready to accept and process feedback. That's not to say that every comment received in a feedback session is valid, or even useful. The trick is always to pick out the most important comments from readers, most important for the story, and most useful because they reveal how my unexpected blinds spot have affected the reader.