Sometimes things work out--but other times they don't. And knowing when to hold and when to fold is a great management skill that I still get wrong sometimes, but it's a huge asset in your arsenal to know when things are just not working out.
That could be from a person, it could be from an account, it could be from a project that you just can't get right. Whatever it is, there's a fine line between between being relentless and dogged and flailing the proverbial dead horse (sorry, gross analogy).
Know when it's time to cut and move on. Even if it's saying goodbye to someone you really value and care about. They'll appreciate it in the long term and so will you--and you'll both be better for it.