If I set a goal to run 5km everyday and one day I run 4km I feel like I've failed. Somehow not meeting the goal invalidates any positive feelings of running the 4km.
I think a lot of side projects have an implicit, unstated goal of "finishing" or "releasing". This is setting yourself up for the same trap as the runner.
It also helps if instead of "run for 5km" I think of the destination "run to the supermarket, to get some cake".
With side-projects is not "add 3 features". It's "add feature A, which then enables B, C, D functionalities".
Also, the more you work on something, the more questions you answer and even more new questions show up. By doing something on your project, you make progress, you unveil the previously unkown. I like thinking of implementing something not as getting closer to the finish line, but as expanding the horizon and seeing more into the distance, instead or looking at a close, dense fog wall of "what could have been".