Remember that failure is part of the process of successful running. Performance is a roller coaster; to think otherwise is irrational and will cause you much stress and discouragement. Lighten up on yourself. Ups and downs can be expected. The performance of most serious runners fluctuates by the week. You win some, you lose some; some days you're hot, some days you're not. Don't fight with yourself when failure, the teacher, pays an unexpected visit. Open up to learning from it.