What am I doing wrong here?

Oh yes please. Definitely needing some sort of nitrogen based nutrient any of the "grow" labeled ones will do...even an "all purpose " would be ok. Just something to bring the N back up. You'll lose some leaves along the way into flowering but this is a little more dramatic than that. Some leaves will yellow with lack of light at the bottom but it's usually slow and mine rarely turn that full limp yellow. Do the leaves pull off with the slightest tug? If so then almost certainly a need for nitrogen. Which is ok in small doses throughout the majority of flowering and medium doses throughout veg cycle. 👍 It should be an easy fix though so no worries.