A frown is considered negative and it's most likely became natural because you've felt bad, angry, sad or whatever for most of the time. When you're upset your body language shows it and it becomes a habit the more you feel that way. Now finally you've realized this and I think it's time for a change.
First, loosen up and have your 'natural' face on. Now take a look in the mirror and ask yourself, how does this person feel?
Now you probably know what a 'natural' happy face looks like. It's like someone has this glow on his face and looks happy eventhough he's not smiling. Now, think about the things that make you happy: your favourite music, pictures, moments, friends. Just get yourself filled up with joy.
Now take another look in the mirror, maybe your face has already changed to a more happy expression but maybe it didn't. Now I want you to make a face that goes congruent with how you feel, a happy face. No fake celebrity smiles, just the face that glows of joy. You're happy and your face should show it.
Of course this isn't a way to make your facial expression be more happy all a sudden, but the more you think about changing your face into a happy face, the more it becomes natural. It's like changing your body posture but instead you change your face.
I do this alot myself. I have a natural negative face myself, because to be honoust I'm not the happiest guy in the world or atleast I was. The more you realize these things, the more you want to change it. It's made me enjoy the little things in life ALOT more and people notice you being more happy in general. Putting up a happy face eventually makes you feel happy just like a confident body posture makes you feel more confident, weird isn't it?
Hope I helped, good luck with everything
