How painful is childbirth?

I have a huge phobia of pregnancy and childbirth. I am slowly conquering my pregnancy fears but almost throw up when I even think about childbirth. How bad is it? I had my kneecap smashed years ago and it was absolutely excruciating- I fainted from the pain and threw up repeatedly whenever I came round. Is it really as bad or worse than that? If so, there's not a chance in hell I'm doing it.