Your stash is growing, that's always good...right?? LOL
I know, I have the same issue sometimes. I'll go along fine, but once I reach a certain point, I love interest.
One thing that keeps me on track though is having a goal. Right now, I am finishing a GB for another site, and that has a deadline. Unfortunately, I've reached that point I always hate...the decals. So many fiddly little pieces to deal with. Honestly, the building and painting are done...my favorite parts.
I also have an older model on the bench to, which I gave up on earlier. I'm determined to finish it this time, so that (at least) I can say it's done.
I try to find things I want to build to begin with, and then try and plan out my builds accordingly, so I don't get too bored with any one type of model.
The other thing that I try to do is to try new things, processes, etc. That, normally, keeps my interest up.
If that doesn't work, giving models as gifts will keep you motivated.