Oh, we have all been there. I can think of a few things that will hopefully spark your interest.
1. Watch a movie or youtube clip that contains your subject.
2. Model another medium. For instance, I'm an armor builder, so I would build a plane.
3. Go to a local hobby club or an IMPS event. I guarantee after seeing other's works, you can't help but want to hit the bench.
4. Read a good related book
I was suffering though what you were going through for sure. I decided that tracks and wheels were getting tedious, so I started work on my dormant Zero. Well the passion started to flare up and this weekend I added 4 more props to my stash at the IMPS show. I still love armor modeling, and I'm working hard to finish my Abrams, but now I have a nice balance in the stash, so when I burn out on one, I have another medium to pick me up.