Just tell her and don’t feed into her emotions when she breaks down. My husband and I were the same, I let depression get the better of me and I never knew how it wore on him until he told me. I got upset at first, but when he didn’t try to sooth me and brought it up again and again I had to look at myself. Where was I failing my spouse? Being married isn’t a free pass to immaturity, it’s important to flatly state that wants/ needs/expectations are not being met. I respect that my husband didn’t just let me go on the way I was because bad behavior would have blossomed into relationship breaking habits.
Good luck with this, there’s no need to be cruel but you also don’t need to coddle.