That's one of the reasons why I like the pomodoro technique. You can always narrow even the most complex issues into smaller pieces that can be accomplished in a short sprint.
A pattern I'm recently noticing for myself is that even if a task doesn't feel "overwhelming" or "nebulous" per se, it seems to be easier to motivate myself to knock out a small task than a large one.
Rinse a dish. Open a letter. Rename a variable. Root cause a bug. Rough out the skeleton for a feature. Write a unit test. Download an installer. Make a commit. Email someone.
I'm noting there's a difference between small tasks and a small task. Big TODO lists may not be nebulous - I find them quite handy for ensuring a complete and thorough job is done, and for project progress tracking - but they're still big, potentially overwhelming huge things. And if not overwhelming, something that I'm not going to finish today anyways - it won't hurt to wait until tomorrow...
Instead of more TODO lists, I should invest in having a TODO item.
With this constraint, what once looked like an overwhelming and nebulous task now seems manageable and concrete.