To Kara, kryptonite is the feeling of ice prickling at her skin like the sensation of freezing in space(but that had not mattered because Kara was at peace then, ready). It's the darkness that consumes her vision before she passes out, so like the deep dark abyss of space(twenty four years of it). It's the feeling of failure that seized her as her baby cousin almost died at the hands of his best friend on live tv(I'm so sorry ieiu, ukr). It's the weakness like waking up in a cage and then again on a lab table like some experiment(and she's reminded of all those movies Alex made her watch that first year on Earth, movies with not quite doctors and not quite scientists and aliens and all too willing scalpels). It's the glint of a sword poking out of a trembling chest(did it see the realest parts of Astra's heart as it passed through sinew and muscle?). It's the feeling of unbridled rage that burnt through her veins and turned her into the monster she sometimes worries still lurks beneath the surface(did the people of National City ever truly forgive her for that? Did she?).
It's the expression of agony on Aunt Astra's face as emerald acid tore through her being, one of the strongest woman Kara knew brought to her knees(and did Sam Lane sleep easy at night consoling himself at the thought that her aunt was just another faceless bogeyman he'd saved the world from?). It's the sound of her own sobs as her aunt's body drew it's last breath, the sickly green illuminating her pale features(did Alex dream that face every night as well?). It's the niggling thought that Rhea, Lillian, Corben will kill her before she can save everyone, that she's a failure(because if she cant even protect Kal then how could she hope to save everyone else?). It's the smug look on Maxwell Lord's face like he knew how much her skin crawled at the sight of him, at the reminder of red k and Bizzaro and Myriad-saving bombs that would save most but not all like that was okay). It's the sensation like it's ripping off parts of her piece by piece(because all it's done is take, take, take)
Kryptonite is the moments where Kara is her weakest, useless and incapable, a burden(why else would Kal dump her with the first family he could find?). How do you convince someone that kryptonite is more than just a gun? How does she convince Lena that kryptonite is the feeling of an explosion at her back and a weight in her arms and a panic that it might not be enough this time, she might not be enough(and oh Rao she cant afford to be weak when she has Lena in her arms, Lena who's good and kind and everything and nothing you'd expect of a Luthor. Lena, who she thinks could be a forever and always, an I do). How can you explain all that, when the closest comparison you can think of is hellfire?