The only way to get faster is to complete interval repeats and tempo sessions, but let's be honest: running hard hurts.
Each hard workout is a mental test: Will you stay tough or wimp out? Finding the motivation to gut out a hard workout is exponentially harder when you're not having a good day. But those really crappy days are sometimes the most important workouts or races to get through—or rather, "gut through."
Certainly you never want to push through a workout or race when you feel physical pain, dizziness, experience chest pains, or are suffering from symptoms of heat-related illness (heavy sweating, high body temperature, weakness, elevated pulse, nausea, fainting).
But it's also important that you don't throw in the towel just because you're feeling tired. There will be plenty of days where you're feeling out of the groove, and on these days, it's crucial to remain focused. Even if you're unable to run the splits you hoped, you'll still reap physical benefits, and gutting it out will make you a mentally tougher athlete.
Sometimes the workouts to be most proud of aren't the ones that are your fastest, but rather, the ones you pushed through when you felt the worst.
Survive the crappy days, and just think how much better you'll run when your legs do feel that extra bounce.
How do you gut through when you feel like your legs have gone AWOL on you?
Chuck the Watch
If your splits are way off from the get-go, do yourself a favor and just forget about the times. Do your intervals, but run purely off of effort. Your body will still get physiological benefits from the workout, despite the numbers you see on your watch. This gets rid of adding even more stress and frustration from seeing slow splits, and keeps you focused on running hard. Don't give your brain an extra "out" by saying, "The times are slow, so why bother?"
More: 10 Ways to Run Simple