i just replaced mine and had the same problem - because water gathers on top of the bolt the upper part of the thread rust away making it very hard to remove it (normal direction).
try spraying a suitable thin oil on top of the bolt and use a good quality tool with an extension bar to move the bolt forward and back again.
It will take ages before it's fully removed and will make screachy noises but at the end it will come out....
clean the tread and screw it back in with a lot of grease on top - better order a new bolt.