Okay here goes.

I wish I had a mirror,
for memories fade away.
to only once look back
on yesterday's today.

^^^ In that stanza in the second line, your wishing your had a mirror for memories to fade away. May want to but "for memories to fade away. Will be more understandable.

To hold it in my hand,
to look, to see, to cry.
To wonder who I am
so I'd never wonder why.

^^^ That stanza I love a lot. Just some what of a powerful stanza of not understanding who you are.

To shine it on my soul,
and hope to see my heart.
To hold it there forever,
until it falls apart.

^^^ This stanza really fit together. Flowed real well.

To reflect it on the earth
and undo all that's wrong.
To shine it on all people
and show them they belong.

^^^ That is a very well written stanza. Loved how you put it in words of it in more of wanting to let people understand who they are and not only you find out who you truly are.

I must lock it in a box
and throw away the key.
To keep it there forever,
for eternity.

^^^ Great way to end the poem, This was a very well written poem over all.