When you work in a team, you make a decision and then everyone must be clear on what the next steps are.
When you work by yourself, however, things can start to be vague. You have to imagine a project and take it through to completion on your own.
And when we zoom into the process of actually doing the work, nothing happens in a neat and tidy way. We might carve out some time to focus like what happens in the Project Lab, but when the timer goes off we are interrupted.
To add more colour to this issue, there are also ideas about how well you will remember things based on being interrupted, which falls under the Zeigarnik effect:
In psychology, the Zeigarnik effect, named after Lithuanian-Soviet psychologist Bluma Zeigarnik, occurs when an activity that has been interrupted may be more readily recalled. It postulates that people remember unfinished or interrupted tasks better than completed tasks. - Wikipedia
Yet the long and short of this message is simple: Keep a notepad by you when you work and write down what’s going on.
Then if you get interrupted, you have some reference of what’s happening when you sit down again.
Zooming out on a project level, keep track of your project’s progress using a more substantial log with one of the systems outlined in this post.