I know that feeling, and I had it not only before my first birth, but before the second and third, too - and now I feel that way again (it prompted me to buy a birth pool a few days ago and pay for 2 day shipping, actually - I suddenly said "what am I thinking? I'm going to pass up possible pain relief just to save money
? Absurd!"). Well, the first time it was more like you describe, and the subsequent times, more like it suddenly came back to me just how difficult it really is, since the memories had been suppressed until then. I've had three homebirths already, never had any tearing or other problems from them, and can probably expect this one to be "easier" than the first, at least, but I still find myself wanting to run away, just a bit!
Hmm, that's probably not the most encouraging thing to hear from a BTDT mom, though... I'm sorry.
Well, there's also this: you can
do it, and you'll probably say it was worth it in the end. Plus, most moms I know (myself included), say the actual crowning/birth part is not the hardest (although you can kind of take that two ways, I guess...).