The same thing happened in high school when I would buy a new album: "I only know two of these songs! And they're both on Side 1! How am I ever going to get the whole album under my belt? How long will it take for me to know all the songs? Augh! Why did I buy this album? I should have just bought the greatest hits, I can't DO this!"
Or, you know, when I start a new relationship: "Who is this person! In what way will he disappoint me? This can't really be as fun as I think it is! I must be co-dependent!"
It doesn't happen when I start a work of fiction. Those start in an orgy of joy over some amazing paragraph I just came up with, most likely in the shower. It's three pages later, like clockwork, when I say, "Oh! God! Come on! What the hell am I writing? I don't know how to structure this. How long is a chapter supposed to be? This isn't shaped right. It's boring. I can't. I just can't, stop it, don't make me!"
Isn't it fun to be a writer? Barrels of monkeys AND laughs. I'm telling you.