I feel your pain OP. I agree this probably isn't the best place for support.
I think it is a kind of grief. My younger son is severely disabled and I get waves of intense and crippling regret and sadness that will follow me my entire life of what we have lost, what will never be. That feels like the feelings I had when my parents died. A feeling it will never be OK again. I guess we can't use that word. Like my son, no one has died, I need to pretend to be grateful and shove down the feelings as there's always someone worse off.
Your feeling sad as that chapter has gone, you can't get it back and things will never be the same again. Ever. Any they won't be. Childhood is officially over and gone.
I'm getting by cleaning my eldest room and feeling very sad. About everything really. I know that the next phase of his life I will forever be a side note in and that's hard to accept.
But it's how it's meant to be. You don't get to keep them. I won't say be grateful as my disabled son will never to university but thats pure bullshit. Your child isn't my son and I'm not you. Be sad, cry. I hear this feeling passes. If all else fails push the feelings down and power through and put them aside to mull over at a later date.
Go clean their room, mutter about how filthy it is and see a glimpse of life not being the carer, cook, pa, taxi driver, ATM etc as I'm sure like my fresher, he has sides that piss me right off. I'm focusing on that right now. My bil said I need to think about my next chapter. I don't want to. I want to go back ten years and stay there forever