I can tell you from experience that Miranda rights won't work. It's only applicable after they arrest and process you. If they question after that point and don't read your mirandas rights, then you have a case. But anything you say before technically getting arrested is admissible in court.

Next time, don't say anything. Never admit your guilty, that's for the court to decide, police only enforce the law, they don't impose sentences. There's various avenues lawyers can use, but if you tie their hands by admitting it, there's not many options they can use.

There's always a public defender. That's better than none at all.