If I stop for a cake&coffee break,or longer,I too have to make a mental effort to get revved up again.
I reckon that I am demotivated by the knowledge that I'll have an attack of café legs( I always,always do
).
I've employed various strategies of mind games & the one I have found most constructive is to regard the whole ride as being the sum of several shorter rides & consider each section as another ride successfully completed.
An ability to adopt the " hello Brain this is Legs......taking over.Over" type of mantra helps.
Riding in the company of others helps too.Especially those who do MTFU.This particular affliction can be contagious if exposed to it sufficiently.
Have a word with the bloke (Matt iirc) in the Bike Shack in Stone.He may know of others locally whose M.O. would suit you.
There is also The Velo Store in Emerald Way who may be of use to you.