Time to remodel your bathroom. If you happen to have one of those lovely avacado or powder blue bathtubs, here's your opportunity to make a change.
Tile showers leak for several reasons. Here's some suggestions to make one last longer than most.
DO NOT USE GREENBOARD behind your tile.
Most types of grout are not waterproof. Most are not even water resistant. You HAVE TO seal your grout with a silicone sealer every 6 months or better.
If you are seeing the leak in the closet, chances are, you will have to replace some studs suffering from dryrot.
Personally, I don't like tile shower and bath enclosures. They are not maintenance free, they cost more than fiberglass, and they are harder to clean. I'm lucky enough to have both types in my house.
If remodeling your bathroom is overwhelming, consider getting some quotes from some local contractors. I get all my leads through a website called www.improvenet.com. you may have some luck there in finding a prequalified contractor who actually wants to talk to you.