So the tasks are: making a choice to sit down and work, and then deciding what to start working on next.

I think that, rather than consciously forcing yourself to do these things it's better to replace the explicit decision process with a habitual process that you can more easily begin, and which results in producing the same outcomes.

To do my PhD writing I take my laptop down to a local coffeeshop and sit there for one to two hours of solid writing. It's become habit. I go to the coffeeshop, I write. My body/brain knows that's what I do there, so there's nothing to fight.

If I try to write when I'm at home, it's much harder. (It also helps that I don't have internet access while I'm at the coffeeshop). Now, instead of making a conscious decision to "start writing" I make a conscious decision to "go to the coffeeshop". Much easier.

To choose what to write about next, I don't sit there and think, "hmmm, what should I write about next? what are my options?". I just start writing. Essentially whatever I'm thinking I write down. I'll have a lot what what I've written last time and write down whatever comes to mind as I'm reading it. What to do next soon suggests itself.

If it doesn't, I just continue to think on paper (or in the text editor), and I always find I end up working on something sutiable. I don't have to explicitly make the effort to think "what should I do next?". I just start writing (which is a lot easier to do), and it soon works itself out.

There is some initial effort to build the habit but I don't think this is any harder than motivating yourself in the first place, and you only have to pay the cost a fixed number of times.