What causes depersonalization to last so long? Like some people have it for years and other people have it for weeks?

Answer 1

depersonalization

https://www.google.com/search?q=depersonalization&...

I am sure you can find the answer in some of the above sites.  Since you know exactly what you are looking for, you will find the answer rather quickly.

Answer 2

 The only reason Depersonalization lasts longer than it should is that it gets stuck in a feedback loop with the anxiety that caused it in the first place.

Answer 3

It is the environment you live in the rest and relaxation is next.