Over the years I have noticed in myself and younger colleagues, particularly those teaching a course for the first time, the tendency to get bogged down in endlessly fine-tuning lecture notes, power point presentations, and assignments.
Some problems with this are:
1. the time could better spent cutting out material and introducing new methods such as peer-instruction teaching
2. teaching is only part of your job and you need to be carefully not to neglect research, supervision, and "admin"
3. you can always improve it next year
So it isn't perfect! Live with it!