Answer:
I can be sweet or sour. I do not rhyme with any other word
While you were folding a letter you got me. Usually you will do whatever it takes to avoid me but now you can't help but find me.
Used left or right I get to travel over cobblestone or gravel. Used up I vie for sweet success used down I cause men great duress.
I can go through glass without breaking it. What am I?
The more holes you cover the lower I go.
What is always coming but never arrives?