Don't store things in your head
There’s certain people with impeccable memory, I’m surely not one of them. It was when I was reading the book GRIP by Rick Pastoor that I realized why it’s bad to keep things in your head: they stay in your head and you’re reminded of them (even if you don’t need it). On one hand you’d say this is a very handy feature, you’ll (hopefully) be remembered when you’re in the super market to buy the milk you needed for a few days already, but on the other hand there’ll also be plenty of times you’ll be reminded of it when it’s actually not that useful, such as when trying to sleep. I’m sure you can think of an example when this has happened in the last week.
To overcome this problem and prevent your brain from getting overcrowded, it’s good to store things somewhere else and know where to find them. For example, when people ask me to do things I immediately put them down in my todo list (in Todoist). If I don’t have my phone near me, I usually at least write it down on a post it note or something to process later. This way, my head doesn’t need to remember to send that one colleague a certain paper, but can instead focus on more important things. Make sure the barrier for this is very, very low, so it’s as easy as possible to get things out of your head. I have the Todoist widget right on the home screen of my phone and adding a task, is just a few clicks away.
When you’re starting with using a todo list, just start with the basics. Don’t bother just yet about all the fancy features, such as tags, reminders, dates, priorities etc. Just use it to keep a list of things that pop up in your mind. You’ll see that as soon as you start using this and be strict on yourself to put things in there, you’ll be able to let things go a bit more, because you know they’re in your todo list.
I highly recommend Todoist, it’s cross-platform, has a good free version and had tons of features (tags, priorities, projects, etc.) for you to use once you get to the feeling of needing those.