Nothing because I think when I die I can't take anything with me and I am used to losing things already.
Tokyo nailed it.
Breathe deep and slow.
Forgive myself for being imperfect.
...Maybe go buy myself a package of cookies and let it go...
You can be angry at yourself or happy with yourself, or sad, or whatever. Your stuff isn't coming back. So you might as well not make yourself miserable, because it's pointless.
Used to hurt a lot, but what can you do, what's gone is gone.
It depends on the circumstances, what the item was, was it a gift etc.