Key is probably worn OK. But get a can of silicon releasing oil [do you guys have WD-40 or equivalent?] The lock tumblers are stuck with dirt.
Use a can with the squirter nozzle. Force it into the lock, squirt in all directions. Use your key to see if you are getting movement.
Be very careful if doing this with an original chrome on brass key!
A steel key is less likely to break in the lock. Don't use enough force that might shear the key in the lock. If you have a spare key that is less worn, use that.
Pump in more oil than you think is sensible. It will free it. That is what the locksmith did with mine.
Bon chance
Joe
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