I think beginnings are always hard and more stressful especially when some of our anxieties (being late, I have a hard time with that too) show up to "slow us down".

I think you're extremely brave to try the "real" chamber group, I wouldn't have the nerve. Can you talk to your teacher about it and maybe practice some of the pieces during your time there at your lesson so you're more comfortable with them, faster?

I guess I'd give myself a certain length of time to try it out and see if I could get it to work. I volunteered to be part of a panel this summer and give a talk about myself and, sure enough, in the beginning when I was starting off I forgot what I was saying and came to a screeching halt within 3 sentences. But I recovered! and everything came out okay and I didn't die :-) and I feel the better for having done it.