Forgotten

[syntheticsunset]

One of the things that separates great writing from mediocre writing is the "show, don't tell" rule. With that in mind, I'm going to make some suggestions below. Feel free to use what you like and ignore the rest.

Forgotten,
Like the way the sky forgets the sun at 1 AM,
You forgot me. This is redundant with line 1

I was replaced.
You traded in your coal for a diamond,
Leaving it to fade into the wind without a trace. What has wind got to do with coal or diamond?
Just like you left me. This is an example of telling. You just showed us with the coal/diamond metaphor. No need to hammer it home.

What once was a treasure
Is now merely rubbish "merely" doesn't add much. Beware modifiers (adjectives and adverbs). Often a good noun is much better than a mediocre noun modified by an adjective.
That you tossed away.
Just like you threw me away. Another example of telling, this line is unnecessary after the garbage metaphor.

Forgotten, redundant
Like the penny left in the pocket of an old pair of jeans, In this line, "old" is another modifier that you can probably live without
You forgot me. you've said this

A good way to improve your poem is to take all the parts you think are good, throw the rest away and then rewrite from there. For example, you could take out some of the lines above and end up with a tighter poem like this:

You forgot me,
the way the sky forgets the sun at night,
traded in your coal for diamonds, shining bright.
Once your treasure, now your trash,
I'm but a penny lost in the pocket of your memory.