I can relate.
I have made peace with it like this:

I can't ever truly "know" what happened. It is highly unlikely there will ever be actual real evidence of my/their memory being validated. If I wait for some kind of external validation before I allow myself to give validation to my own self-parts, then I may not ever be able to do that.

On the other hand abusers are invested in children not telling the truth to others. To do this they invalidate their victims, make them doubt themselves, their own reality, their own knowledge, their own perceptions, their own truth etc etc. Then throw in some DID where we have spent our whole lives denying our own truth and splitting it off from ourselves then of course we are naturally not going to trust our own inner experiences. We have spent our whole lives actively denying avoiding repressing forgetting our own reality. We have conditioned ourselves to believe it is not real.

So we find ourselves in this situation where we have no outside source of The Truth and an internal experience that is telling us it is real but we don't believe it possibly could be. On top of this, it is really unlikely there will ever be an external source that can confirm The Truth.

What can we do?

Maybe we just have to trust that we entered this world as a child of God perfect in every way. That perfect human being that we once were then had experiences.... hurtful experiences... sometimes soul destroying experiences.
But in the beginning there was just that child of God, pure and true.

Remembering this helps me listen to my selves.
Maybe some part of this will help you too.