I'm overwhelmed with guilt over a friend who disappeared even though the cops know what really happened
That must haunt you, but why do you feel guilty?
I found out that my friend was cheating on me. And I didn't even notice it.
It is okay to feel bad when something happens to a friend but you shouldn't blame yourself especially since you never noticed. It might have happened anyways.
She's dead and I'm still upset about this.
It's normal to be upset after a friend dies. It might take some time, but the pain does eventually fade some.