I suggest you rent a tile saw. You can then cut small slivers off the end of a full tile matching the angle of the intersecting wall. Fill the gap until there is a small enough space where 100% silicone caulk will cover it. I had the same problem when I tiled my shower. Used one of those score and snap tile cutters. It worked o.k. until you need a thin piece. You could also glue a plastic corner molding on the gap.