Rule #1: Keep moving forward despite your moods. You can't allow achievement to depend on mood. Grown-ups have to get the job done no matter what. If Rule #1 fails, apply...
Rule #2: When things get too tough, take a vacation. Say, "I need two (or three, whatever) days off." At the end of the allotted time, you're likely to feel much better.
Rule #3: The difficulty you're experiencing is normal--and necessary. Sometimes writers have a hard time with stress simply because they haven't recognized that stress is necessary. It's not par for the course--it is the course.
Rule #4: Don't doubt yourself. Identify the negative influence that has caused your resolve to falter. Lack of confidence is for all of us the greatest enemy. But the successful person manages to move forward despite his/her lack of self-confidence. Self-confidence increases when you continue to act (in this case, write) with no regard for your insecurities.
Rule #5: Associate with positive people, and stop associating with negative people. Nothing is more helpful than a positive support group, and nothing more damaging than constant negative reinforcement from "friends" and family.