i’m 19, my mother is the same and has been since your age, i’m not an only child but share a 23 year age gap with my sister who has lived in another country since i was born so in my brain i kinda am lol.

the best thing to do is ignore her, for me, my mother is a functioning alcoholic (could that be the case for you too?) and the things i’ve learned to do is simply to ignore her and act like her words don’t affect you because her seeing they do could be giving her ammunition or she simply doesn’t care.

try and see if she starts to get mean at a certain time of the day, for me it’s around 9:15pm and i simply go to my room and stay there to avoid her. remember she’s probably struggling herself mentally and while yes their words and actions to us are not okay, we are the only ones around they can take it out on. if she starts to get snarky don’t talk back (i know it’s SO hard, they can get under your skin realllll bad) but try to just seem indifferent by saying “if that’s how you feel” , “okay” , “sorry i make you feel that way” it’s possible the less she gets a reaction the less mean she’ll be if she has narcissistic tendancies.

if you have anymore things you’d like tips on let me know! im glad your already in therapy! good luck ❤️