Feeling like I wasted 6 months on a side project that won't benefit me
If you learned something --either in terms of programming, or in terms of life --then it wasn't wasted time.  Next time you're in a situation like this, you'll know to say "no" up front.

This may seem like a glib answer, but it isn't intended that way.  As long as you don't keep making the same mistakes over and over, you are moving forward in life.