what to do, when you can't meditate? I am meditating on a daily basis since a couple of months. I noticed having ups and downs in my meditation, probably mostly caused by the amount of sati or (on the other side) stress I had during the day. 

But I am also experiencing sittings, where I am just overwhelmed by the hindrances and not even able to recognize them properly.

What is the best way to deal with these extreme states of mind (in meditation)?
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.