Can you sue for false arrest based on your own confession? Plenty of people confessed to crimes they didn’t commit before the internet, and many got arrested. Did any of them triumph in such a suit?
Not sure of the details however, if you tell a cop you were selling heroin yesterday, they can’t or shouldn’t be able to arrest or charge you.
So I’m thinking that the logic would apply if you admitted to a crime after the fact, even if that crime was much more serious.