I want to quit my job, help

hey there.. I am wokring in a retail store.. and I really want to quit. Apperently im bad with being on time and today i had to call in sick and go to the docters because of an infection. I honestly dont see myself working there anymore and i want to give in my 2 weeks notice.. would a docters note even matter? At this point i know i am messing up and obiously i look bad in the retail world.. but i want to go back to school. I want to continue and get my Ged.. I dont want to work in the retail industry, I hate it.
I travel on bike too and thinking about biking up the hills is tiring. Im really stuck.. I want to be successfull and happy but i am not. It makes me even more sad to see how broke i am .. I feel like leaving this job will do me good.. for my sanity and health and putting my education is a priority but being broke scares me.. Im actually feeling depressed cause my bf has a full time job and he is now in the beverage dept.
I feel really bad that i had a bad connection with my boss because of my tardiness. and the way how i left was horrible. I should not have been treated that way.. SOmetimes i do think about ending my life because i seem to not connect with people that greatly especially compared to my bf.. or anyone else in that matter..
It just .. I guess I am that rotten apple in the family tree. And it really kills me..
This is a tough time for me.