My PhD defense is next week and I am having negative thoughts about my work and knowledge. Any advice on how to tackle this?
Take a deep breath. You will be OK. You're suffering from impostor syndrome. 

PhD defenses are traditional formalities. Your advisor wouldn't let you schedule yours if they didn't think you ready.

The examiners are more likely to want to know what you did than  theory from your first year you have forgotten.

When you pass your defense come back here and tell us about it.