i've decided to quit my job for my own mental well-being and he's practically begging me to stay which is not something i feel comfortable doing. i don't know how to just assertively tell him i do not want to work here. 

he told me i can take a few days off but i am starting a new medication that will take weeks for my mood to level out so this isn't something i can take a couple days off with and it'll all be better.

i also find it so selfish of him to tell me this when in my resignation letter i told him it's due to mental health issues. that is such a large range of topics including suicide because for all he knows i could be on the verge of it and he's asking me to stay.