The hardest thing to learn was for me how to plan. Not to execute what I have planned, but to make so epic to-do list and to schedule it so thoroughly, so I'm really capable of completing all tasks on schedule date.
Why was it so difficult?
To plan something you need to think about it. Often we find ourselves caught in daily routine: go to work, check e-mails, answer them, do some stuff, go back home, eat dinner, go to sleep, go to work...
But even in that routine that everyone knows you have some vague spots like "do some stuff". And that stuff, especially at work but also at home can be "watch some funny cat videos, procrastinate and panic when you find out that you missed another day" or it can be "find time to do project you've always postponed and to watch funny cat videos with pride, that you've finally completed that tough task".
It's really all about planning your day.
I learned how important it was when I started to use task management apps. I could of course set up tasks due tomorrow and named "do something" but I realized, that completing the tasks which has a name on the date they supposed to be done makes me feel good.
Currently I use the task management app (Itemz) that helped me set a habit of planning also for another reason. It has trophies that you get for using the app and doing your job. The trophies are exciting and sometimes unexpected so to get them I almost overused this app, but it also helped in developing really good habits of planning and keeping my calendar clean :)