Hi Unreal,
Welcome to NeuroTalk.
I'm pretty sure that there can be quite a number of reasons for someone to experience depersonalization and/or derealization. [I have experienced it myself at times due to anxiety.] It sounds as if you've been dealing with these feelings for a very long time.
If you've had medical checkups to see if they were seizure related and for other physical causes and they've all come back clear, then if I was in your situation I'd probably ask to speak with a Psychologist about it all. Bear in mind though that there may be physical causes that were never investigated. Certain drugs and medications may also make people feel that way as well - cannabis is just one example.
Migraines can be a factor as well and it doesn't seem to be widely known that not all migraines involve headache. When you say you have no seizures, I wonder if you've ever had investigations such as an EEG.
You could check out the
Epilepsy Forum if you like. The members there might have some ideas for you as well.
Sorry to prattle on so much. I hope you find some answers because feeling that way all the time might surely have consequences in the way that you live your life.
take care there.