Exactly the same thing happened to me with my Black and Decker GX635 mower.

I repaired mine with a narrow steel strip some self tapping screws and hot
metal Glue. Once fixed I taped a wooden stop on the main handle that stops
the switch from being over stressed. I do think that it is a design fault as
the travel on the switch after the required contact is far too much and it
is difficult if not impossible to not over stress it especially if you are
cutting the grass on a bank.

The repair is now in it`s second grass cutting season.