It is not necessary for you to write in it everyday, but it helps to record various insights you gain as you go about living your life. Instead of using a big notebook, you might use a small note pad that you can keep in your purse or pocket for easy access to record your thoughts as they occur to you. Whichever method you choose, what's most important is that you write your thoughts down. It will help you know what's in your heart. Be honest with yourself by paying attention to your actions.
Actions speak louder than words, and they always tell the truth.
On the other hand, if you say you're not good at a certain job, but your actions say otherwise, that's also important. You can use it to make more beneficial choices in your life. By being honest with yourself based on your previous actions, your actions moving forward will be based on truth instead of just what you tell yourself. Take time out during the day for quiet time to listen to your inner voice. I call this inner voice the voice of God.