Always write down every smallest achievement. So that whenever you feel blue you can read it and feel better.
You can also vent out, later when you'll read it you might find it funny how you were angry about such a stupid thing.
Maintaining a diary also helps you organize your thoughts.
You can also write down your dreams whatever it is. Our dreams keep on changing. For example when we were kids we wanted to be a doctor who saves lives or an astronaut who gets to go on moon and space. I wanted to be an air hostess. But as we grow up they keep on changing. Write down the date on which you decide what you want to be every time.
Maintaining a diary in a way also helps you to start loving writing. It'll help you make your handwriting better.
You can note down small small ideas that pop into your brain suddenly. We tend to forget such ideas real quick.
If you have an exam coming up you can note down what you want to do or how you want to prepare for it and set your goals accordingly. And mark things that are done and that are left to do.
Maintaining a diary also helps you decrease your screen time which is very important for your health .
It also helps you keep your mental health stable as you can let out all the things that are stuck in your mind. It'll feel like a weight is off you chest.
Writing down things also boosts your memory.
It also inspires creativity inside you.
The last and the most important point. You do not have to worry about anything while you are writing in you diary, since the only person going to reed it is you. Be carefree and just believe in yourself.. Thank You.