Sounds like time for a Dremel tool and cutoff wheel assuming they will fit in the area where you need to cut the portion of the bolt with the nut on it off. Otherwise, drill and EZ-Out. If not that, cut out and replace the entire assembly. It happens sometimes. Sucks, but it happens.
What happened to you happens to all of us sooner or later.
FWIW, next time, squirt with a 50/50 mix of acetone and transmission fluid prior to attempting to disassemble. You can buy the squirt cans for less than $10 on Amazon.