I have similar feelings to Andy, and I'm experiencing it now, with the Mitchell.
Bags of enthusiasm and motivation to start off with, and looking forward to getting the finished model into the planned diorama setting.
But as the build continues to drag on - mainly due to health issues over the past months, I'll admit - I just want to get it done, and move on to other things.
I tend to really 'immerse myself' in the subject, focusing and concentrating on almost nothing else apart from the details, history, background etc of the subject being modelled at the time, so I've vowed to myself that, in the future, I'll restrict this practice to a few 'special' subjects, and try to do more OOB builds, or as near OOB as possible.