Ooh, and my t said it can help to not get caught up in the whats and hows and whys of it. because that doesn't change it. She says to notice it, name it and act - do things that will shift it.
So like if you notice you feel symptoms of depersonalization then acknowledge it and then go straight to doing grounding things that will help it. that way you can start to shift it instead of getting stuck in it deeper and deeper.