Help! Where's my mojo?

I was sat at my workbench yesterday looking at my latest project - two 1/35 scale Russian mountain troops scaling a rock face, and I realised I had no motivation to finish the piece!  Initially I was excited - I found the piece of rock in a friend's garden.  As soon as I saw it, I knew what I could do with it!

I wanted two soldiers one on the lower ledge, one above - grappling hooks, crampons - the works.

I made a start, buying the kit from which I would take only two soldiers...

I selected the two I wanted and made them so they would look right on the rock...

But that's pretty much as far as I've got!  It's almost as if, having brought the idea into reality, the spark has gone out.

I've lost my mojo!

I have a stack of kits on my bench, vehicles, troops and accessories - all with ideas for scenes and dioramas - but nothing excites me at the moment.  The buzz of a new build is not there.  

I suspect the after effects of my aggressive radiotherapy in December are interfering with my ability to focus and concentrate.  The consultant did say I'll be feeling tired and lethargic for up to a year afterwards - but this is more like apathy.  I sat there for about 15 minutes yesterday, and felt sad, sad because I had no desire to build, to continue, to finish what I'd started.  I felt uninspired by the piece - and I thought "That's OK.  I'll postpone this one and make a start on something else!"  But I didn't even look at the stack of new kits because in my gut I knew that's not what I wanted to do.

I just wanted to sag into the sofa and watch TV.

So.  What's going on?  Why do I feel this lack of oomph, and how do I get my mojo back?

What happened to the passion I once felt, where I said to my wife "Just popping to the shed - I'll be back in a bit!" and two hours later I'm still unable to tear myself away?

Do I wait for the wanting-to-build feeling to return naturally, or do I force myself to sit and make something?  I need help!