No, you didn’t do good. But, Mustang’s don’t lease well. That’s why I bought mine. Your selling price is pretty high as well though. You can get at least $3k off.
Then you should have bought a GT lol. I have an Ecoboost. It’s 3 years old and a fine car. It makes absolutely no sense leasing a Mustang or Camaro. You can own the car for like $50 more a month.
Leasing it is not a bad idea, especially it being your first car. If you have an accident (your fault or not), it lowers your vehicle value considerably so only leasing it means you have a guaranteed way out. Just keep it in good condition, and in 3 years if you still like the car you can buy it or turn it in and move on to the next vehicle.