Finishing a huge, long-term project is a great feeling. But just as you're feeling the high of getting something done, you also might feel the need to start criticising yourself over all the things you didn't do right. Blogger Sarah Peck calls this a "project hangover".
For a lot of us, the project hangover is inevitable, but Peck suggests the best thing you can do is prepare yourself for it:
It is important to also remember that a project life cycle has within it the natural hangover phase; the point at which you are so sick of hearing or thinking about it any more that it's time to put the pencils down, pin the work up, step back, get feedback, and take a short rest. An overnight to reconsider. And preparing for this lethargic state, in my experience, helps wonders.
If you find yourself beating yourself over minor problems after you finish a project, the idea that you can prepare yourself for that might be helpful in making it less painful. As Peck also points out, you're so deeply invested in a project by its end point that you take criticism, especially self-criticism, very seriously. Preparation might help keep that under control.
Project Hangovers and Self-Criticism [Sarah Peck]