first your question....I never had to cope with forgetting large portions of my life. put simply forgetting was my normal. as a person with DID it was just my normal to have parts of my life "forgotten" or in psych terms dissociated. then after integration everything that was my alters became me, so again I didnt have to cope with forgetting beyond the normal standards because all the alters memories became mine again.

now that Im integrated I do have to say my memory on a normal standard has changed. example before I was integrated an alter would remember where I put something, go get it and everything was fine. after integration it is frustrating at times to not be able to find something right away.

I have had to learn that no one in this world has a perfect Utopian memory where they remember every important and unimportant part of their lives and daily living.

what helped me in this was taking a class on how the brain functions (physical parts of the brain, brain waves, how it stores and discards memories, the 5 senses and so on)

maybe you can ask inside to your alters to find out what those memories are and also take a class or visit your local library and do some research on how the brain works with storing memories.

the second part of your post my own treatment providers called believing in something that isnt real (ie that its the past) PTSD or psychosis (hallucinations / delusional belief) depending upon other accompanying symptoms/ problems. it was called dissociative in me only when another alter was in control, who was stuck in the past time frame and event that happened to them, every single time one of my alters was in control they reacted, behaved, thoughts, ... everything that made them who and what they were was as if the same event was happening, that it was the same year, same clothing, hair styles everything was the same. it wasnt something that just happened once. it was every time.

example Beth (not actual alters name due to profanity filter) was my alter who was stuck in time. her sense of agency was to deal with one set of problems (wont post exact problem due to possible trigger for others), everything she was, said and did was that day, that fear, being hyper vigilant, keep us safe at all costs. every single time beth was in control she would run off somewhere, trying to find that safe place, safe people and never being able to find it/ them.

my treatment provider and I took the approach that she was there for a reason and she was doing what ever her sense of agency was. Since she was one stuck in time we knew she was not the type of alter that could learn, grow, evolve. If she happened to come out / take control during therapy my therapist took time to offer any comforts she would be willing to accept and waited her out,

eventually another alter or I stopped dissociating so that my therapist and I or another alter could talk about what triggered Beth to take control. then we worked on ways that we could be less triggered in case it happened again (here where I am when one dissociates its a reaction to something just like getting angry is the reaction to someone doing or saying something you dont like. )

my point if this is a dissociative problem for you maybe you can work with a mental health treatment provider that can help you to find the trigger so that you or your alters dont have to feel like its still the past.

in me when the problem was not dissociative we did medication adjustments that would help me to not have my delusional belief that it was still the past. the meds also helped with the PTSD problem when it was that too.