One experience that has impacted me the most was the passing of my grandmother.
To say "one experience" implies "one of several." But when you say "most," it is superlative.. so there is only one.. So you should write, "The experience that..."
Let's make it so that the stuff on each side of the semi-colon could be a complete sentence:
It was the hardest thing I have ever gone through; seeing someone so loving and caring go through such torture, the fact that
and there
is I could do
you nothing to help her made the situation even worse.
This is highly significant to me because I lived with her my whole life and she taught me so much about life.---- This sentence is too vague. At the end of the first paragraph, give a sentence that contains the message of the whole essay.
:-)