I second that, run and learn!
Always try and find something constructive even out of a "bad" run, for example...your legs felt really heavy but you could still finish a 5k!
Next time your out you will know that even if everything feels wrong you can still finish a 5k so it will help you along.
If everything else seems fine eg.,diet, water, fuel..etc, check your form, are you wasting excess energy? Learn to run efficiently.
The other really backwards advice I can give you is when you feel like everything is going wrong sometimes the only thing to do is keep running! 
Swap a training run for a "fun" run, ditch the watch, forget about time and splits and just run!
No plan, no time, fast, slow, road, trail it doesn't matter just go and play!
And the most important thing I can ever pass on.........Don't forget to SMILE! 