A shower pan leaking is a problem. Usually when an older shower leaks to the ceiling below, it is the shower pan below the tile that’s failed. Any sealers or caulking you use from inside the shower will only temporarily stop the leak.
Under the tile is a lead pan that covers the floor and up the walls about 10-inches that is watertight to the drain. Any water that weeps through the tile and grout is trapped by the pan and down into the drain. The pan will usually fail at the walls or where it is connected to the drain.
The proper repair is to remove the bottom two course of tile and the floor of your shower, and replace the old pan with a new PVC pan and a new drain assembly. Usually the trap and some of the drain line is replaced at the same time. New tile is installed over the pan with the proper drainage.
Some insurance companies pay part of the repair for damage done to the home from the leakage. Consult with your agent for possible relief.