My PhD defense is next week and I am having negative thoughts about my work and knowledge. Any advice on how to tackle this? I am presenting my PhD thesis in front of my committee next week and I feel that I have just not done enough. My work is simple and I also feel that I have poor theoretical foundations and am going to be an embarrassment when quizzed by my committee.

I am not feeling confident about presenting my research and I am finding it extremely difficult pushing away negative thoughts like (I feel I have not done enough or I am not theoretically strong enough for doing my PhD on my research field).

I am revising theoretical stuff (related to my thesis work) that I studied in my first year of PhD, but I am not able to push out the feeling of guilt and the impending doom.

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.