Hindsight is a great thing. So, I suppose, is patience and the ability to wait. When it comes to writing, or being a writer, there's a very direct link; stories and the writing in them always look worse in hindsight, and get better if you wait before you edit.
Writing always looks worse in hindsight - there's a lot in that statement. If you've written something, then it's OK to be proud of it. Heck, if you finish a book, it would be odd not to be proud. And even if you know it's not perfect, you think you're getting somewhere. There are even scenes, or sentences, or particular phrases, that are pretty damn good. You're not half bad at this. You can write! You're a master!
But take that writing, leave it - ooh, a couple of weeks? A few months? - and go back to it. It's not pretty. It's like rewatching an old cartoon from your childhood and seeing the horror, the cliche, the nonsense of the stories in all their garish 1980s glory.
I loved Captain Planet as a kid. I couldn't see anything wrong with it. Heroes from all over the world uniting through awesome magic to save the earth and beat the bad guys! Yeah!
I watched an episode with some friends when I was at university. Bad Idea. The inane stories, the cheesy, throwaway characters, the morals crowbarred in to the episodes - oh, man. We swore off Captain Planet and preferred to keep our childhood memories alive.
And why was there never a European there, either? And ... Power of Heart? Please. Everyone knows that was the worst of all the Powers. Pff.
Anyway - we looked back at something that had been so good in our minds and had turned out to be so bad. Writing can do that. Going back to read stories I wrote and just staring in mute horror at the sheer levels of ineptitude isn't exactly an ego booster, but it does help those stories get better, in the end. When you can see flaws you can fix them. If you're too wrapped up in the moment, you tend to lose sight of the point of what you're doing. It's not just to get the story down, it's to do it in a way that's readable.
And also not to waste time watching old cartoons. It doesn't end well.