There is a poem that I found written in my dad's old belongings after
he past away 15 years ago. It is titled "What is the Stars" but I
don't know if he wrote it or if it exists out there by someone else.

The begining of it goes like this:
"Forget the tears, the groundless fears
Excruciating pain.
I?m in control, both mind and soul,
Just how I can?t explain.

The last line reads, "I Know What is the Stars".

Can you tell me who wrote this poem please?

There are many, many obscure poems that are not mentioned online, so I
can't be certain that this isn't a published poem from some
lesser-known source, but I used ten different search engines, and
found no trace of the poem.

Your dad may very well have written this himself. If I were you, I
would make this assumption.

I ofen looked up at the sky an' assed meself the question -- what is
the stars, what is the stars? (Sean O'Casey)

This is the closest I could find--knowing what I know about O'Casey,
the nuance of the poem you have is in some way most definitely related
to him.  This search could help if you wanted to go through it more
closely:

://www.google.com/search?q=%22sean+o%27casey%22+%22what+is+the+stars%22&hl=en&lr=&ie=UTF-8&oe=UTF-8&start=10&sa=N