Self-definition is a tricky thing. Who we see ourselves as helps guide us, keeps us making good moral decisions and is an anchor to us. It also can be a bit of a crutch, and lets us hold onto things for too long. For a long time, a big part of my self-definition was framed by my job; I took pride in being a successful, strong creative professional. But at some point I started believing it was me, that I needed it, and that's when the relationship got a bit sticky.
The same thing is true of relationships. Break-ups are hard because not only to you end a close bond with someone you love, but you also break the view of yourself. You are no longer the couple. You are no longer Jack and Diane. The myth and history you had been building goes up in a puff of smoke, and you're left standing there, in your sweat pants with a pint of Ben and Jerry's in your hand, just you and what's left of your own personal history.
Don't get me wrong. I'm not saying this experience is all bad. I'm just saying that where ever you are in life, it's important to keep grounded in you.