I was bullied in high school so bad it drove me to planting a fake bomb threat note on a bully to get rid of him
That's pretty extreme, why did you do that?
When you're getting bullied, you get desperate.
What you did was very serious in terms of legality. I get that being bullied affected your thinking, but you went too far.
I did it so I'd be sent to a mental hospital.
You need to outright ask someone for help. What you did could ruin someone's life by putting them in jail for something they never did. Be up front and honest, take the blame for the bomb threat and tell one of your teachers or staff that you need help.