i am looking for the title of s short story set on mercury, where all the inhabitants live entire lives in just a few days. i belive the protagonist is named Sim, and I am quite sure the love of his life was named Lite. Thanks!

Sounds like "Frost and Fire" which can be found in "R is for Rocket."

In countries outside the U. S., it may have appeared under the original title, "The Creatures that Time Forgot."