What is the best means for overcoming anxiety and derealization?

Psychotherapy. With medications if you need them to 'get over the hump' and into a treatment that, literally or not, slowly exposes you to the very thing that makes you anxious. Anxiety and derealization should be checked out medically first. One thing that can be helpful is to use sensation to 'anchor' you. If you pay attention to your senses (direct representatives of 'reality'), it's harder to dissociate.