I'm a bit of a defeatist, so I get a bit paranoid that my script is terrible and poorly thought out.
I'd got about 1/3 of the way through my script and then convinced myself that the plot hadn't been established soon enough, but after a bit of feedback from this site I thought "Well, yeah I guess they're right".
Now that I'm getting close to finishing my first script, I'm starting to doubt whether the structure of my script is good enough. I've been on holiday today and had plenty of time to panic about whether my script does work. I suppose it has actually helped in a way, because I've thought of a few areas in my script that I need to expand on.
I suppose the main problem is that, as mentioned above, I'm a defeatist and if my first draft isn't a work of art then I'm just not good enough. I have managed to stick it out with my current script though. It's perhaps the first time I've ever stuck with something after getting pangs of paranoia early on.