Check out www.johnbridge.com. they'll at least help you decide the best way to fix this, even if you don't want to do or can't fix it yourself.
In order to prevent leaking, there needs to be a vapor barrier behind the walls that laps over the flange of the pan. With what you have, it sounds like there is too much deflection, and when you are standing in the shower, the floor deflects, pulling the pan away from the walls. Caulk only can do so much. If you let it dry out, cleaned out the caulk, then put enough weight in the thing while caulking, you'd have the biggest gap. Let it cure, then remove the weight, it MIGHT last longer, but it is not constructed well.
Note that the tile and grout is not totally waterproof, so water can migrate through it (that is why the vapor barrier is required), accumulate and do some nasty things. Any cracks in the grout or tile make liquid water available behind the walls. It needs to go somewhere.
The best solution may be demolish and rebuild. I'd be worried about how the old floor was repaired...my guess is you have too much deflection. It would need to be addressed for any long-term reliable fix.