Or perhaps you just research another little overlooked niche and start on your next simple little, low-maintenance money-generating site.
It can be done. The main ingredient needed is persistence. Been there, done that, and I have a very nice lifestyle to prove it.
The $300 a month target is very conservative. If that's all you earn, you've done something "wrong". You haven't chosen profitable keywords carefully enough, you haven't built enough attractive, keyword-rich pages, you haven't learned the basics of optimizing pages for search engines, or you haven't attracted enough good quality links to your site.