I think switching interests come from not seeing what else you can gain from current interest. When it comes to losing interest in completing a project, it probably comes from your brain thinking it's a dead end with no end anywhere in sight.

To maintain interest in something your brain needs to you see improving in it / getting better at it over time.

Also you have to convince yourself that there's a worthwhile future at the end of the tunnel, and that it's not a waste of time.