I talked to hundreds of people...
Tertia did a post for me about my situation.
The comments were very supportive coming from both sides. Of course, the negative ones are a bit hard to take. A number of people said I should have thought more about this before agreeing. The thing is, I had been considering doing another donation. I was struggling with it because of all the reasons that make me so reluctant now, but I had been considering it in the abstract. When the clinic called, I had two impulses. First was that I had been considering it, so I could probably handle it. The second was that I basically had no right to deny these people a full genetic sibling to their first child. If the clinic hadn't mentioned that part of it when we talked, I probably would have said no. Even going into this with some reservations, I had no idea my reaction would be so strong after agreeing. Even as I debated the posibility of another donation in my head, I never thought I would have this kind of anxiety.
I think toward the end of the last donation, I realized a few things. First, I am really terrified that my family will find out about this. I can see it ruining a relationship there. This is my decision, not their business, and I feel fine keeping it from them. I just want it kept. The other much stronger problem is that one of these children/parents might try to track me down. My anonymity in this process is the absolute most important thing to me. Just the fact that there are some documented records out there that could reveal this information is awful to think about.
So I still don't know what I'm going to do, but I guess I'd better hurry up and make a decision.