It seems you and I are destined to talk tonight. I have lived with dissociation for more years than you have been alive. It's good to know you are aware you are doing it. Believe it or not, that is a good sign. Many people with PTSD dissociate.
I suggest you write down in a journal each time that happens. Write down the date of the event, as well as the date you are remembering. Take them with you to your therapist the next time you see him/her and they can help you process what is going on. Something has awakened that memory in you. Something in your current time zone. Once you can figure out what that trigger was, you can learn how to dissipate it before you go back in time.
Just like everything else with PTSD, it won't happen over night, but it will happen. I learned a strange thing a number of years back. The only time that happens is when your mind feels the body is safe enough to handle what ever that memory is.
You are not alone any more, surviving_it_all.