Help pls

I am half a tank of gas. I am a fish tank with half of its maximum capacity of water. I am a theatre with half of its maximum crowd within it. I am a pessimist. I am a 8 GB USB drive with 4 GB of storage used. I am a vase with half of the water I am able to carry in it. I am viewed differently by many, but I am one perspective. What am I?