When someone has something, but then slowly loses it because of something they did wrong, you can say that they "let it slip away":

I had a chance to write my own comic book series, but I let it slip away by not working quickly enough.

You can "let" a person "slip away" too. This usually means that your relationship with that person was ruined:

She was such a great girl, but I let her slip away. That's something I'll always regret.