Further to my post on accepting your personal best as great, one more point.
Life's not a straight line.
Pick anything in life - education, relationships, work - they'll all have their ups and downs. Sometimes for obvious reasons, but sometimes for no good reason at all. Life includes its fair share of inexplicable funks.
As with any real funk, the most important thing is recognizing it for what it is and soldiering on through to the other side.
Running's a great example. From my office I have 3 set running routes. One's 4km, another 5km and the last 7km with the amount of time I have at hand in part determining which route I take.
A few weeks ago I ran my best 4km. I felt strong, breathing was easy, my strides natural. All in all feeling really great allowed me to push. Fast forward a week later, same route, same time of day and within 1km I knew I was in for a crummy run. I hurt, my breathing was laboured, and I spent the last 3km mentally forcing myself to finish. It was anything but a fast and easy run.
Was I upset that my run stunk and my time was markedly slower than just a week earlier? Nope. I was proud as hell I finished because that run. It wasn't about speed. It was simply about finishing despite not really wanting to, and doing the best I could while feeling crummy.
Whatever your aim, whatever you're trying to accomplish, some runs will feel great, and some won't. Your job (and yes, I know I'm a broken record)?
Do your best, and love it!