i understand what you are talking about, i get foggy a lot, sometimes it lasts all day with brief interludes for food! I tend to find it happens after a few consecutive busy days. i have to take a duvet day at least twice a week to prevent pressure sores on my bum as i can't walk, so i try and take them on the foggy days, so i don't have to explain why i can't do things that day! playing games and chat rooms on the computer in the morning leave me very foggy and wipes me out for the day, so now i only visit these in the evenings or on duvet days when i am in bed anyway.
i too get frustrated as although i am foggy and my body does not co-operate in things like sitting up or getting dressed, my brain still knows i should be doing things. but experience has tought me that if i fight the fog and get up/do things then i will end up breaking down in tears for no reason other than i am exhausted or frustrated because everything i try to do either goes wrong or i drop/sipll etc.
be kind to yourself. it is the only way, be patient, you will be more productive in the long term if you rest when needed than if you struggle on fighting against the fog.

foggy on its own could be a number of things, as can zoning out. dissociation can come in many forms too, so how dissociation affects one person may be different than how it affects another. best to work with your T on becoming aware of your episodes, once you are aware of them you can then work on recognising the triggers and warning signs, then you can work on how to stop them before they start.

hope this helps