Answer:
Open me, and you can't see me without a mirror. Close me and you can't see me at all. What am I?
I am the second L in LOL.
Break me and I am better, immediately set and harder to be broken again. What am I?
My second is performed by my first. A thief by the marks of my whole might be caught. What am I?
In buckles or lace I help set the pace. The farther you go the thinner I grow.
What is always coming but never arrives?