what to do, when you can't meditate?
You are certainly not alone. Sounds like what you describe is a situation I have been in many times. 

In order to find more balance, I would stay sitting still and try to remember that I could just be with the tension, fuzzy lack of clarity, confusion, anger, desire for the practice to go my way and whatever else arises. Usually this would lead to a better practice session.

This problem is mainly about unbalanced   faculties. A good teacher can help yogis balance the faculties. A good teacher can be a wonderful help when yogis are tangled up like this.