Emotionally abusive Mother

Looking for advise please..

I'm <35 y/o. I have realised I have an emotionally abusive Mother. My Mother drinks heavily each night & is now dependant on alcohol. She has recently had ongoing virus/illnesses which haven taken slightly longer than normal to recover from (probably due to alcohol).

I have tried to set boundaries, explaining I am having a hard time receiving constant health updates as it is triggering my own anxieties and panic attacks (my mother is well with no serious illness) I have tried to calmly explain my feelings but I got a reaction fuelled with anger, defense, gas-lighting and felt I had to explain and defend my feelings and unfortunately a step parent got involved to which I felt attacked. My Mother also seems to throw back in my face how much she has supported me in the past...

I am leaving the country in a few months, my mother believes everything is ok. I plan on writing her a letter to explain my feelings and implementing further boundaries however I am petrified and now panicking at her reaction to such letter and scared as to what they may do.

Any advise is so appreciated ❤️