Time is the one resource we'll never get more of, yet somehow, it's one of the first things we forget when planning a project. If we know we're doing the right thing, then this is just what it takes, right? Maybe. But my advice would be to try and dig a little deeper.
Learning to make yourself scarce
Instead of just asking "What do I need to do to get this done?", add "today" to that. And you really need to mean it. To the point of, "If I work on this for the next 8 or so hours, and leave, and never touch this again. How do I make sure I've created something of value?"
To be clear, this is not about shipping two months' work within a day or two. This is about maintaining a steady velocity and progress over time. Needs change, so do our products, and I might tell you we're starting a multi-year project today only for it to be binned a few months down the road. Now it's up to us to decide whether those few months will be all for nothing, or if we'll still have things to show for.
Accepting that time is fixed helps us remember the scope can be flexible too. We become more critical: is this really essential or is it just noise? Instead of trying to do everything, we have to pick our battles carefully and deliberately. And there are personal upsides to it too! How does working on a single PR for days or weeks on end make you feel? How often have you realized at the end of it that it could've and probably should've been multiple changes?
Shaping it up
This is not groundbreaking stuff. In fact, people have been doing this successfully for years and it's one of the core principles behind the Shape Up methodology. It's the same thing, except on a slightly larger timescale.
Forcing ourselves to make our work fit the time and not the other way around makes it easier to determine priorities and focus on the most important parts. Thinking on a scale of days or weeks is much more predictable than months or years. Shipping often also gives everyone a much bigger sense of accomplishment.
And in my opinion, this translates very well to the individual level and planning your day too.
If you can't get it all done today, chances are there's more to it. Take a minute to break it down and note all individual issues you're dealing with. Regardless of what methodology you might be using, not only will this make your personal updates much more credible, but it also makes it much easier for anyone else to jump in and help move things along. This is how you build trust and confidence.
After all, software engineering is a team sport.