Hi jillster,
I've been out for a few weeks, so just saw this thread.
I'm one of the moms that pops on here from time to time and tries to give the view from "the other side".

I honestly think it is just the "permanent" that has you in such a panic. If you have been happy this long without children, and had never planned on having any - nor ever really felt the pull to have any; I think it is the fear of not having the
option anymore that is scaring you so badly.
Even when you talk about making this decision, you are not actually mentioning the fact that you might want a baby - but that you might want to have the option to have a baby. (Does that distinction make sense?)
I have 2 children w/ my first husband, then divorced and remarried and had a 3rd child with my current husband. Even though my oldest son has Asperger's (a type of Autism) I am very maternal - and babies especially bring the mothering out in me. But my last son nearly destroyed my kidneys and I went through horrible post-partum depression, and I wound up needing a hysterectomy a year later. I knew intellectually that I could not have any more children anyway, my husband had already had a vasectomy 1 month after our son was born because my OB said getting pregnant again would be very dangerous for me. But losing my uterus and ovaries was like the nail in the coffin for me. I mourned my "womanhood".
Even though I knew I
couldn't have any more children (or I should say shouldn't), I still had the ability to. But losing that option was what really hit home with me.