Here's an interesting story related to this phrase:
So the next time you find yourself staring at a name on a phone screen, watching a plane disappear into the clouds, or standing on the wrong side of a closing door—remember the phrase. Whisper it. Weep if you must. And then keep going, because the act of reaching, even when you know you cannot touch, is what makes you human. tooi kimi ni boku wa todokanai
However, I believe you might be referring to a different title: "Tooi Kimi ni Boku wa Todokanai" is also the title of a light novel series written by Sakae Esuno and illustrated by Ruri Miyama. The series was later adapted into a manga and an anime film. Here's an interesting story related to this phrase: