When I fall in love with a personal project, it happens to me too. The important factor is that I don't have anyone else to explain myself to, no one blocking my changes, I own the whole project from the bottom all the way to the top - it's basically just a giant positive feedback loop.

I checked my stats and over the last six weeks I have dedicated my spare time (of which there has been more than usual, but I have a family and work 9-5) to my current project, and I've averaged 7 commits per day with 59308 lines added, 22903 deleted.

I am feeling a little burnt out now, but fortunately the code is essentially done and all that's left is a little mop up and writing a blog post.

If you have a list of hundreds of APIs to implement, you can easily skip the ones that are hard to (properly) implement in the first months. You also may choose to keep TODOs in your code that indicate 'minor' issues whose implementation is tricky, or that may already be correct, but 'just' need checking. For example, if a call that throws must not make any partial changes before throwing, you may whip out something that likely is correct, but postpone the actual verification of that fact.