Most Helpful Girl
That's a tough one. You might need to shoulder the bulk of the reasons on this one. If she's genuinely suicidal then you need to make it not her fault (even though it kind of totally is) and it needs to be more about you and you need to make yourself the bad guy.
Also, you need to contact her parents, school officials and potentially law enforcement (law enforcement can put her on suicide watch) and you need to tell them BEFORE you break up that you're planning on it and are concerned for how she'll respond.
Then you just have to do it. Rip off the band aid. And then don't be her shoulder to cry on. Suicidal or not, she has to handle a break up the same way as everyone else. She'll have an informed support system to be her shoulder instead of you.