... even though the day may seem grey...
Simply put there are three things that define your state of mind and how you feel:
1. The body's movements. "Emotion" comes from "emovere" which means "move out". How you move is how you feel. F.ex. move "as if" you are happy if you want to feel happy.
2. What do you focus on? What thoughts do you choose to focus on and thereby increasing their importance and impact. Do you f.ex. think thoughts that make you happy or sad?
3. What is your mental model of the world? The world to you, is it f.ex. a place for opportunities or limitations? Your mental model of the world is sort of a "frame" that defines what you allow yourself to think within that frame.
These three all interact with each other.
For more effect: Define and write down a result you want, f.ex. being happy at least 60% of the time. (Often we are feeling down 60% of the time.) Make your choices, actions and thoughts according to that desired result.
Enjoy your day!