Relationship Sell-by-Date

I get that all relationships go through ups and downs but when is it time to call it quits? Is it when it stops being exciting? Is it when you start feeling like you are being taken for granted? Is it when you can’t remember the last time you had sex? Or when, for no real reason at all, you wake up and the magic is gone?
I ask because I’m the kind of person who holds onto relationships way past their sell-by-date. I hate saying goodbye because it’s so hard. The person I’m dating, if it’s a proper (longer than 6 months) relationship has invariably become my best friend.
How on earth do you say goodbye to one of those?
When you date for a fairly long time the person you’re dating becomes your closest ally, your confidante. They are included in almost all the good memories you have of a certain time period. You can’t just remove them from your memory or erase them from your relationship history book.
I suppose that however you slice it saying goodbye is, unfortunately, going to be traumatic. I know that you have to experience the downs to enjoy the ups but, I for one, am not the biggest fan of the downs. Perhaps it’s not just me? Maybe it’s human nature to want to shield ourselves from the trauma? Maybe that why so many of us subconsciously decide that it’s OK to live in Relationship Neverland for as long as possible?