I have had a very similar experience with constantly questioning the quality of my work. What I found helpful was if the project is something I really want to do, then I tell myself any mistakes are part of the learning process.

For example I'm writing my thesis. It can be really discouraging when after writing a paragraph on research results I notice that I made a mistake in analyzing my data. I've struggled with feeling like I've wasted my time since I have to redo the data analysis and rewrite the paragraph. However telling myself I was always going to make this mistake, and I must be learning since now I can see the mistake has helped me stay more motivated.