Esa día vs Eso día
Ok....so día is a masculine noun. Does that mean that we use eso with it instead of esa? Desde eso día he tratando ser alegre pero.....sin ella..... ya no se como hacerlo.
If you need help remembering the forms of the demonstrative adjectives don't forget that there is also a great reference section here that succinctly covers these topics.
Also don't confuse the masculine forms of demonstrative adjectives/pronouns (which take the -e ending) with the neuter form of demonstrative pronouns (which take the -o ending). I have often been guilty of this in the past before I finally drilled and drilled and drilled and drilled and...it into my head.
In any case, here's a link to the related reference articles in case you are interested:
But your sentence should be:
"Desde ese día he tratado de estar alegre pero.....sin ella..... ya no sé cómo hacerlo".