The best thing you can do is return the coving to the wall. this is, run the top of the coving so that it is level with the door frame then do an external angle and put a small piece between the wall and the coving like a very small external joint.
Part of your problem is the fact that you probably sealed the bath whise it was empty. then when you fill it with water AND get in you have a lot of extra weight pulling the sides away from the wall. So by all means use the Bath strip that fits into the corner BUT GET INTO THE BATH WHEN YOU FIT IT!!...
