I've been in similar situations. You're going along and not much changes but all of the sudden you plunge into a deep depression and wondering what in the world happened. I think the reason could go a little deeper than the breakup (not to say that a breakup isn't painful or insignificant.) When it comes to trauma there are big T's(traumas-car accident, feeling your life is threatened etc) and little t's(conflicts with family*, breakups, school trouble etc). One big T could send you into a downward spiral but multiple little t's stacked on top of each other could send you into a spiral too.

I hope that made sense. Basically, I'm wondering if you have had smaller stressors over time that piled on top of each other that caused the depression. Just a thought.

I hope this helps somewhat!

*I got the examples of little t's from a Psychology Today article written by Elyssa Barbash Ph.D.