What type of bike are we dealing with?
please! (it really helps to see what we are dealing with.)
I'm going to guess it is a beach cruiser of some sort with a coaster brake. Possibly a Huffy.
Did you open up the hole in the rear sprocket, or just mount it and throw away the dust cap on the hub? (been there done that.)
do you ride fast and jam on the brakes to "try" to stop quickly?
stopping these things with a coaster brake generates a LOT of heat which goes straight to the bearings which overheat and self destruct.
buying a new Hub is where i am at right now as well, and was advised that this is a good direction to go.
or you can buy a complete wheel with a Shimano. (There are other more expensive, and/or better options, but this seems to be the best bang for the buck.)