My method of handling existential thoughts is to acknowledge the absurdity of the question I am asking myself. It's odd that we question our own existence while in the phase of DP/DR, but we do, and we know that it's because of said disorder and this feeling of unreality - so I believe the best method to overcome them is to acknowledge that they are nothing more than an anxious mind searching for unattainable answers. Try to go beyond the initial worry that follows an existential thought - try to acknowledge that it's just an anxious mind doing what it does best; messing with you. Don't let it win.