Do Momentum Building Work
When you have a large project to get into it can sometimes take a lot of activation energy to really get into it. This might mean that the first part of your day gets spent trying to wrap your head around what you need to do to tackle the big thing on your list.
One way around this is to not start with the big thing. Have some "momentum building" work to do. This means have some tasks that have a clear approach and you can complete quickly to start with. These could be house-keeping tasks like cleaning up all your open browser tabs or sending out a meeting invite or reading a document that you know you need to look at.
You could also spend the last part of your day figuring out what you would do if you were going to work for another hour or so. Write this down, along with all the context you'd need to get going and save this work for the first task of the next day.
The idea is to quickly get going and feel productive. Once you're in that mindset, it should be easier to tackle larger, less defined work.