No matter what caulk you take, it will peel off within a few months. Caulking is an old and outdated method of sealing grout or corners as it is not water resistant. It tends to peel off in wet conditions and shrink in dry settings. I also had a similar problem in my shower. A friend told me to use Sentura. It is an epoxy resin filler that is water proof. I used sentura to seal my shower corners and also to repair a minute hairline fracture in my shower tile. The results are outstanding. The best part is that it can be shaded in more than 40 colors and so it can match with the surface easily.