Sometimes, there's a task. It's The Task. Maybe it's responding to an email, or paying your credit card bill, or opening up the dissertation file, or taking your trash out. Whatever it is, it's The Task that you know will make you feel better to do it, but also you would pay one bazillion dollars for it to be done and for you to not have to do it.
There is a lot of productivity advice that will say that when you have this task, you should do it first, and they call it "eating the frog." The idea is that if you have to eat a frog at some point during the day, it's better to just get that over with so you can eat more delicious things later without the dread of having to then also eat a frog. Get the hard, tough, maybe gross stuff out of the way, and everything else is crackers and La Croix, or whatever your favorite food is.
In general, this is good advice. It usually feels good to get The Task out of the way, and feeling good at the start of a work session is usually helpful for overall motivation. Once a hard thing is done, you have concrete evidence that you are, in fact, capable of doing hard things.
However, sometimes, you know you have to eat a frog so you just.....avoid breakfast, so to speak. You say that you're going to start with The Task, and then the minutes (hours) creep by and The Task hasn't been started, and the dread builds, so you don't do anything. You just wait to feel up to eating the frog, and keep waiting.
So here's my compromise, a workflow that balances getting tough stuff out of the way and also my own tendency to not always be my strongest in the morning:
When I end the workday, I identify one or two important tasks to start with when I am next at my desk, that will feel like wins to get done.
When I sit down for the next session, I check in with myself and see if I feel like I want to start with those tasks, or if something else has come up that I need to attend to.
If I am starting with a difficult task, I set a timer for one pom. If I haven't started the Frog Task in the first pom, I switch to something else.
I try the Frog Task again when I either a) feel up to it or b) after a hard break (like lunch, or a workout) when coming back to work feels like a new session.
It's great to eat the frog, and it is an awesome way to inject a bit of win right into the beginning of a session. But it isn't so powerful that you should sacrifice the rest of your work day ready to eat that frog. No one tool fits well for everyone, at every energy level, with every kind of task. So experiment with it, and learn when it works for you, so that you can use it on purpose!