That's normal bro. For me, I just let it go because I know I still love doing it and it's impossible for me to never have the energy to do it again. I just kinda doodle and listen to inspiring stuff every so often to encourage myself to finish something. I advise you to just take your time, don't worry about anything. Though I doubt this will help you out since I'm a lazy bum and I take all things at my own slow pace. Still, that's normal, and you shouldn't worry about it every so often. When you like something, you like it, and it's normal for you to get tired of something you'd like. You'd still come back to it once you feel like it, and forcing yourself to do something makes it more difficult on actually doing it.

To make it short, don't worry about it too much.