It's hard to truly say, but I've had several breaking points or near breaking points that it is hard to really pinpoint which one will really do me in for good. In 2010, I had a major breakdown and pretty much just stopped caring whether I lived or died. I was at the peak of my academic studies as I was in university at the time, but due to some conflicts between my professors and academic advisor, my quest for a great academic record was shattered. Things really went downhill since then, my academic records slipped, while still very strong, it isn't like what it was in my first two years of university. Also, my social life is terrible, had little to no friends and no one really speaks to me, hangs out with me, invites me out for events, etc. I also have a lifelong condition called Aspergers which fucks me up socially and also anything to do with interacting with people and social skills. It doesn't help that I've developed social anxiety during my teenage years either.
Then fast forward about 4-5 years, in 2014, I had a terrible wage job (which I had to take just to support myself - to not go homeless) and parents who aren't supportive at the time. The owner of the restaurant was a real bitch and she would be openly hostile towards me. I quit the job after about 2 months and it just made me suicidal. Things didn't really get much better when I went back to school in hopes that I will be able to improve my life. Social life was still shit, new babbage along with old babbage, then I had some people who were concerned about me and some school admins decide to question me (luckily nothing came out of it other than just a small meeting - but that was enough to make me worry about how it could be much worse), and of course, just barely made it out of school now. Still jobless and not wanting to work shit jobs, ruining my mental health even more, just barely existing right now until the time is right.
So in summary, you could say my breaking points are when social life becomes shit and there is little hope (more like non-existant) for things to get better on that front, then failed dreams that I could not have, shitty life circumstances, job prospects, having to slave away just to exist (if I somehow do find a job and didn't lose it or ruin my mental health), and having to hide my suicidality in order to not get locked up, involuntarily committed, sent to the psych ward, etc. If I get sent there or am just on the verge of being sent there, I believe that would tip me over to finally ctb, along with my current circumstances as well as long term problems that have no solution. As of now, I'm just barely coping and existing.