I honestly just don't know what to do anymore PLEASE help..?

i have a best friend, we'll call her sarah. sarah is the most important person in my life, we've been best friends for almost 7 years, blood couldn't bring us anymore close. i got sick this year and couldn't attend a lot of school first semester which resulted in bein pulled out of my classes and do online courses. sarah, got closer with our other friend, we'll call her alice. so sarah started dating this boy, we'll name him jack. well from the get go i was againest it, jack seemed like a prick and i let her know that. she and all my other friends got mad at me but eventually got over it. wel now the tables have completely turned and everyone has a new opinion of jack, they hate him. sarah barely sees him but she won't break up with him. even though tere's been rumors of him cheating on her (which is true but she doesn't know but eeryone else does.) during the time sarah and jack dated sarah fell into some bad habits. she started smoking cigerettes and smoking weed everyday sometimes even more than twice a day. (keep in mine she's 16) she steals from her mom, lies to everyone, and i've even seen cuts on her wrist. so obviously she's in a bad place, everytime i try to help i get **** on. she doesn't listen to me or anyone else. when i ask alice for advice she ***** everything up and sarah gets mad at me. sarah ignores my calls, texts, everything. according to alice she "doesn't enjoy talking to me." all my best friends won't talk to me and i have no idea why. i haven't talked to any of them in almost a week, i seriously want to die. suicide has crossed my mind every single day. i've gotten really close to comitting it. please, please help me.