At what stage in academia are you most likely to suffer a mental breakdown?
You have given more than enough evidence to show that you need to seek professional help and/or counseling.  The impulse of telling people that you're in trouble is a good one.  Posting anonymously on a site for academic advice is not the way to go: because you clearly need help by those with professional counseling training and very few academics have such training, most people on this site will (and should) decline to give you specific advice.  Please seek help in real life.  You can begin with student counseling services at your university.  If you don't know where to go, google "student counseling [your university]".  

Note: I am leaving this as an answer rather than a comment because I feel strongly that this is "the real answer" for the OP.  The manner in which probability of mental breakdown varies with the stage of one's academic career is not relevant to the OP.  Mental breakdown can happen at all stages of an academic (and nonacademic) career, and rather than looking at age or seniority statistically one should look behaviorally, cognitively and emotionally on a case-by-case basis.