Your character is your legacy.

Every single person leaves a legacy when his/her earthly life is over. Your very existence has an impact on the world whether you intend it to or not. Your character tells who you are. It's what you'll be remembered for. So, what legacy do you want to leave behind ?

Character is your nature. It is who and what you are when no one is observing. In today's world that is we are overly preoccupied with image. The tendency is to worry too much about our reputation and too little about our character. A reputation is ever changing and can be destroyed in a brief moment, but character is built in a over a lifetime and is everlasting.

A good character is the best tombstone. It is by this  that the world will remember you when forget-me-nots are withered. Therefore, build up your character and live in a way to that will carve your name on hearts, and just not not just on marble!

Here are a few notes:

For the first correction, try to read the sentence like this:

"Your very existence has an impact on the world whether you intend your existence to or not.

I could expand this idea even further to show a clearer meaning.  

Your very existence has an impact on the world whether you intend your existence to have an impact or not. 

This is becoming repetitive though, so it's not a good idea to write this way.  Without adding the to you are saying that you intend your existence or you don't intend your existence, which is not what you meant.

The second correction (that is - we are) is needed because "that" refers back to "character", which would make the sentence say that character is overly preoccupied with image.

I changed the comma to a period to avoid a comma splice (two sentences that would need a conjuction or a full stop between them.)

If any other corrections seem confusing, then shoot me a message.