@speccy1 I haven't read the whole thread but, for what they're worth, here are my thoughts.
My motivation to cycle alway goes up and down a bit. I mean, it's bound to; there are so many variables. Weather, time available, family, other hobbies and everything else that life conspires to throw our way. My solution to this is simple. Don't worry about it. Howling wing and freezing fog? No thanks! I only cycle to avoid weighing too huge a number. I'm not racing anymore. I'm luck to get out one night a week.
This night riding has stoked the motivational fires for me slightly this year. A modern light has blown new interest into old routes.
Many people have mentioned mountain biking. I agree, it is that different, whilst still cycling, it might help your interest flare again. You say you are no good at it? Maybe you are trying the wrong sort of mountain biking? It doesn't all have to be gnarly descents and pulling air off the doubles! You are in/near Dartmoor! Throw some waterproofs and a small stove into a backpack and off you go. I used to live down there and some of my best memories are of my brother and I riding the Puffing Billy (as an example) or the endless bridle paths around Princetown and then a pint in the Plume of Feathers.
My bro still lives down there, I'm sure he would jump at the chance of a bit of a ride on the moors. He loves it up there and knows it very well if you fancy a bit of an explore.
Don't sell everything. I have always returned to cycling in the end.