I've been there too and I like that you mention that you feel like you web app is just a series of hacks. I feel that way too and I'm sure others do as well so why do you think that is? In my case it's probably because we see so many other successful people launching and we start thinking of all the things that could go wrong and that just makes us want to start over.
I'll give you an example. I've had this VPS over at Webbynode for months now just sitting there. It was originally going to be one app but now I just set it up as another last night. (writeapp.me in case you're curious). So I set up my LAMP stack, uploaded whatever part of the app I had to far (which isn't much) and proceeded to install a mail server. Well it went well except I screwed something up and I can't access the web interface for it. That one little thing, not having my mail server work perfectly made me think about starting over with a fresh Ubuntu install. Crazy! I think your idea of what it will become when it's finished has a lot to do with this stuff.
I'll give you an example. I've had this VPS over at Webbynode for months now just sitting there. It was originally going to be one app but now I just set it up as another last night. (writeapp.me in case you're curious). So I set up my LAMP stack, uploaded whatever part of the app I had to far (which isn't much) and proceeded to install a mail server. Well it went well except I screwed something up and I can't access the web interface for it. That one little thing, not having my mail server work perfectly made me think about starting over with a fresh Ubuntu install. Crazy! I think your idea of what it will become when it's finished has a lot to do with this stuff.