When it all seemed gloomy, I always saw a light from the darkest side. If you want to use "Dawn" you have to use another example that is related to dawn When it all seemed gloomy, I always saw dawn from the darkness(Or "Shadows", "Night", ect).

I like the beautiful sentiment! Perhaps dropping "a" would sound better, but even without the indefinite article "a", "I always saw dawn from the darkest side" sounds slightly odd to me... You could say, for example, the following to add a little touch of poetry :) Even in the gloom of my darkest hours, I knew that dawn was nigh (or "near").