I just had the same experience with a fire truck. A Simba 8x8, a German truck specifically designed to put out fuel fires at the airport in Frankfort.
I started off OK, the frame and undercarriage were fun, but then I slowed down, to the point I was forcing myself to work on it. I lost interest but didn't know why.
Then it occurred to me, "The thing looks like a child's toy." It looked like a Fisher Price tonka truck. Then I REALLY didn't want to work on it. Then my USS Constitution came in the mail, then I REALLY REALLY didn't want to work on it.
Fortunately, my OCD wouldn't allow me to start one until the other one was finished.
(I won't ever understand you multiple build people. Madness, I say!!)