Not sure you'll ever get that sealed properly. WHen using a drop-in tub as a shower, you need an add-on tiling flange. If one of those is used and installed properly, the lip will keep water inside of the tub. At least along one side, it appears your tile goes up against the edge of the tub and the only seal is caulk.
If you have a removable panel, you may be able to push some mortar underneath the tub to stabilize it so it can't deflect. That would help, then caulk may last longer. You may want to consider adding a full circle shower rod, which should keep most water off the sidewalls. If the caulked areas have any gap, filling it first with foam backer rod, then caulking will also extend the life of the caulk. This forces the caulk into an hour-glass shape so it has a narrower section that flexes better than a block of it. It is less likely to tear off one edge when done that way, plus, you end up using less caulk. The backer rod comes in all sorts of diameters, but you may need to search to find it in the size you need. The big box stores only tend to carry the big stuff, not the smaller diameter stuff you'd likely need.