At first it was just a feeling. A feeling of something false. You didn't just wake up one day and knew what happened. It was a slow development. It came with sticky tentacles, determined to never let you go. You went through your daily life like always. Sometimes you saw the shadow, but it was always small. You never lost the lurking feeling of sadness. Trying to live a normal life and not paying attention to the feeling and the shadow, you gave them the power to blindside you. Sometimes when you looked into the mirror, you felt something starring back at you that wasn't you. There was something behind your eyes you couldn't really catch. One day you realise the shadow was surrounding you and growing into your world. You tried to shake it off but it was futile. The creeping feeling and the shadow were one and the same and they held you tight in their claws. You became afraid of the mirror, afraid of seeing what was growing inside you. Surprised nobody else could see it. Your mask was firm in place. But what if it broke? You wanted nobody to see. You didn't want to be weak. You fought for your life. You gave everything you had to not to lose yourself in the shadow. You fought for weeks but then came the day you found yourself falling right into what was lurking inside you.
(I'm not a native speaker, so please bear all my mistakes.)