there are many different types of alternate personalities with many different reasons /disorders/diseases of why they are there. how and why they disappear depends upon the reasons/disorders/diseases and many other things like medications, therapy, and many other issues...

example
an alter that is there because the person is having a hallucination or delusion or psychosis will go away on its own or with medication.

An alter that is there because the alter is a willfully created (consciously or unconsciously created for reasons like pleasing a treatment provider, wanting to be the disorder, invisable friend and other reasons) will disappear/integrate on their own when the person has no more need for that type of alter.

I could go on and on about the many different kinds of alters and why they would or would not disappear/integrate.. the list is endless and is different for everyone.

only your own treatment provider and your self can say how and why this alter of yours disappeared/ integrated.

what I can tell you is that there is diagnostic criteria that states with the dissociative disorder DID the alter must be a special kind and must take control many times not just in therapy.

here in New York, which is in the USA it is believed alters with DID regardless of whether they are fully formed alters or are what we call fragments, dont just suddenly appear and then disappear. they come into being when someone is under extreme abuse as a young child under the age of 5. then after years of therapy the alters will integrate when their job/purpose/reason for being is no longer there.

other locations may hold other standards and beliefs about DID type alters.

the best thing I can tell you is go according to what ever your own treatment provider says your alter was and what ever your treatment provider says is why that alter just suddenly disappeared/integrated with you.

I have had similar experiences but my forgetfullness was more beyond the normal.. anyone can forget things and can not remember things for a few days including stress, dissociative amnesia, physical amnesia, anemia, sleep deprivation and so much more. with me my forgetfulness met the diagnostic criteria for DID..I would forget things like seeing a relative and the reason I would not remember a family trip to that relatives house was because that relative abused me. I would forget where flash lights were because I was abused with a flashlight. I forgot I ever had a velvet christmas dress because I was abused in that dress. Each one of my forgetfulness went beyond the normal realm of forgetfulness because it was trigger related, and the trigger had to do with the abuse I had dissociated. The same when I would forget days...it want just I forgot those days and I appeared to be different. the reason why I appeared to be different was because something triggered me, which caused me to switch into an alter and then the alter took over for those days. the memory of what happened was still there, it was just inside the alter, once integration happened the memory of the missing days came back to me. other times I would not remember whole years and during therapy it was discovered something related to my abuse triggered me to switch into an alter who decided to move out of the area of the trigger.

I was similar to you as in I had walls built up between my self and my alters the difference was that I didnt consciously create those walls. those walls happened during abuse situations...example when I was in a mine shaft being abused I was too young to understand what was going on and the pain was too much for me to handle, they were adults and I was a child so there was no way I could get away from them.. the result was I dissociated myself to where mentally I was not in the mine shaft mentally I was in a boat floating on water. I didnt create the wall between the physically being abused and the mentally floating in a boat on water. my brain automatically did that with out me being aware of doing so. that wall was the sound and feel of water. from that day on through my whole life that "wall" of water was there in my head. 24/7 I would hear the sound of water and sometimes feel like I was floating on water. this "wall" of water was integrated with me when that alter was integrated with me.

I do set up some walls consciously but it doesnt have to do with my dissociation problems. because of the type of work I do, I have to keep my life and feelings separate from my clients. in order to be the best I can be for my clients I cant be falling apart when they are telling me about their problems/painful events. I cant be thinking of myself and my feelings when I am on the witness stand in helping to prosecute abusers. I also cant be bringing my work home with me. I cant be with my wife while constantly dwelling on my clients, the court cases, abused children and such. the result is the moment I walk out my door to go to work I have to be in my work mode frame of mind and the moment I leave work I need to switch over to my private home mode frame of mind...

I used to think my being able to separate parts of myself this way as the dissociative disorder. but then my therapist explained to me that separating parts of myself and my life is completely normal....people normal and disordered do naturally place walls/ compartmentalize their life. children are one way with their parents, another with the baby sitter another way with their teachers, another way with the substitute teacher, another way with their friends.. adults do the same thing they compartmentalize their life accordingly..they are one way at work, another at home, another way at church, another way with their friends even between friends adults compartmentalize by being one way with this friend and that way with another friend. So I dont worry about it any more.

my suggestion is if these things continue to worry you, contact your treatment provider. they can tell you how to do what we call "grounding" "staying present in the moment" so that this separating of your self and your life doesnt keep happening to extremes that bother you.