There's been great discussion in the "productivity" space about what motivation is and does it really exist.
Some people believe that motivation is a myth. They say that success can be only attributed to one's self-discipline and habits, not some fleeting notion of "motivation". Others deeply believe in motivation as something that fuels work. Something that we need to have to work diligently and in a fulfilling way.
I've struggled with finding motivation to do things many times in the past. Many times I've been in that dark place, where you just can't get yourself to do a single thing. Not out of fear, but out of a lack of motivation.
After all these years of figuring out how to life and how to work, I started shaping my own definition of motivation.
I think it's kind of a mix of both approaches; for me, motivation is a mix of being excited about fulfilling work and self-discipline.
Discipline is needed to start and then continuously work on a single thing at a time to do it well. Yet, no truly meaningful work can be done with just self-discipline. It has to answer to some of your core life values. It has to be exciting, demanding, but at the same time fulfilling.
Whatever motivation is for you, there ain't no sunshine when it's gone.