DESERT CENTER CA - I sense that you are NOT permanently loosing interest, due to the fact that you reached out to others on this forum for our help/input. The fact you did this says to me that just perhaps you sense deep down you love model trains and creating a layout. You may be frustrated and feel a blockage about moving forward with your layout but somehow I think you are not done with it all. IMHO.
There are periods that I don't have any appetite for my trains. At times these periods last for weeks or even months. This has happened many times over the last dozen years. There was even a time I was seriously contemplating selling all my trains due to my interest being at almost "zero" , however, the appetite came back around and it was an immense appetite!! Even after three months of not hardly running or going into my train room and thinking of seriously selling, I promised myself that I'd give it a year .... If the low interest consistently persisted for a year, I would have taken the necessary steps to sell it all OR at least greatly downsize.
Over the years, I've learned to trust that there is an ebb and flow with my layout interest. ( Heck there is an ebb and flow with everything in life ... at least for me ) I don't get particularly panicked whenever my interest wains. I just notice that my interest is at a low point and I don't attach a value judgement to it..... I just notice the low or almost no interest. I trust that the hunger for my layout will come around again and I'll be running trains and working on the layout most everyday perhaps until the next time my interest wains.... and again I will know that my appetite for my train/layout will return.
When my interest is at a low it's usually because other facets of life are demanding my energies. Usually after extremely long busy periods when other areas of my life demand my energies, I crave to come home, let my imagination go and run trains and/or work on the layout.
Sometimes just watching some real world railroad videos ( especially films from the 40s and 50s or some of the OGR layout videos help stimulate my appetite and imagination. The idea of imitating real life railroading does it for me! After all, to me IMHO, its all about imagination and letting our imagination go free.... like I did when I was a young kid with a 4X8 layout on a flat board with no tunnels .... just a loop of track with two remote control switches and one train with 6 freight cars and a caboose. I could get in the "zone" becoming lost in my imagination and time. Hours could go by and they seemed like a half hour. Yes, we adults know that life is not a fairytale, however, it sure is a healthy thing for us just to take a break from the real world and "play" ... and playing trains is a healthy way to take time out and enjoy life IMHO!
Others here on the forum have given some insightful and hopefully helpful comments/ideas regarding your layout and about getting your mojo back for finishing your layout. I'm sure you are mulling those suggestions over. Hey, once you get back at taking on a project .. planning it out etc., you very well may find yourself back in the game big time!
And I realize to that model trains are not the only thing in life. Life offers many wonderful activities and pass times. I wish you good luck on your journey... whichever path you chose to follow.