How do you keep up motivation during individual doctoral research projects in the short run
Perhaps you are just pushing too hard and getting frustrated. If you try to force your mind to work, it can get bogged down in the detail letting you miss seeing the bigger picture and options that might let you go forward.
Often the best solution is to take a break and do something different for an hour or three to let your mind clear itself out. Aerobic exercise, even if mild, is good for this. So is going to the coffee room and chatting with colleagues about trivial stuff (not work). &quot;How about those Mets?&quot; (Sorry NYC allusion). A game of Chess, perhaps.
Many people have the experience that when they awake in the morning they have solved a previously impossible problem in their sleep. The mind works even when you don't push it to do so, and sometimes more efficiently.
I can't guarantee that this is the issue, but I'll guess, at least, that it is more related to frustration than motivation.

My own trick, back then, was fifty mile bike rides with a couple of friends.