"Forty, Counting Down" is a short-story involving time-travel written by Harry Turtledove. It first appeared in the December 1999 issue of Asimov's. This same story is also told in the story Twenty-One, Counting Up but from the protagonist's younger perspective. It is available in the collection Counting Up, Counting Down. In 2000, "Forty, Counting Down" was nominated for the Hugo Award for Best Novella.
In this story set in the near future, 40 year old computer genius Justin Kloster invents a time-machine based on string theory and virtual reality. Using this device, he travels back in time to visit himself when he was 21. It was at this age that he began dating his future wife who later divorced him. As Justin never recuperated from this loss, he decides that this time-machine is his chance to redo his relationship with her with more success. As such, he convinces his younger self to lay low while his older self (who happens to have aged very well) courts his girlfriend. Unfortunately for Justin's plan, he finds that his older self is even less successful than his younger one, and his girlfriend leaves him, much sooner than she did in the original timeline. The older Justin decides to return to his own time in the future, leaving behind a large sum of money. When he awakes in his original time, he discovers that he has successfully founded his own string-theory company, and is happily married to someone else and has several children. He attributes this to his original relationship not becoming as serious, as well as inspiring his younger self and providing him with seed-money to start his own business.