You overtightened the nut and striped the threads, if your really lucky the stud is semi ok and the nut is ruined. put a prybar behind the bracket so you can pry the bracket against the nut. try removing the nut while prying, an impact wrench is best for this. You dont have room for a nut breakers probably, so use a hammer and chisel. aim the chisel along the side of the stud, nut into the stud and remove with as few well placed blows as possible. This means hitting it really hard and well. Or use a cut off wheel and cut again straght in along the threads, use a chisel to finish the job.

If the stud can be chased out with a die, clean up the threads and use a couple of washers to raise the nut onto some clean threads.

Buy a torque wrench and use it.