A quick article with opinions from six people on project killers: 6 Experts Share the #1 Thing That Derails a Project | Smartsheet. Of course, there are six different things listed as "the #1 thing." And there are a few more listed in the comments.
More on time management and multitasking. It's a topic near and dear to what I've been doing for many years.
A quick anecdote from Realization's newsletter on "There is no such thing as good multitasking" and some thoughts around the idea.
An interesting talk from AI researcher Kenneth Stanley on his counter-intuitive discovery/realization that formal goals/objectives can block creativity.
The recommendation? Give it a whirl - try something more focused on the task at hand. And if it doesn't work, email will always be there, like an old habit that you can't break.