stupid shit (vent + need advice pls)

so today i was coming back from school with my friends like i usually do and like recently i've been getting off at a metro stop before the one i usually come back by cuz my bsf's cancer came back recently and like his chemo is right after school like 3 days a week. my school friends (idk if they r tbh) would joke about following me after school to see where im going cuz i never told them. but they actually did it today and i was like spaced out while walking there so i didnt notice. but they followed me into the hospital and like then i saw them. i started fucking hysterically crying and then i was so idk some negative feeling that i locked myself in the toilet and fucked up my thighs so bad and it was literally almost gonna be a year of me not doing it in 6 days. 6 DAYS. and i missed his entire chemo session like idk how i missed 3 entire hours. and he called me later like crying cuz this one was apparently a bad session cuz he was already feeling more crap than he does usually when he wakes up and then he had to do chemo but he was like "oh but you're gonna reach 1 year in a week so that makes me happy." but then he said he threw up like 11 times so and the fact that i wasnt even there makes me feel so awful and disgusting and the worst person cuz i literally left the only good person in my life for something i promised him i would never do again AND he was so excited for me to reach 1 year even though he was feeling like absolute garbage. him mentioning that just makes this feel a 100 times worse. and now i have to tell my bsf about this. but like how do i tell someone with literal cancer that i was destroyed by something so dumb. pls i need advice :)