I think maybe learning better coping skills would help you. Today people seem to have more anxiety than ever before, so we must learn how to deal with it. I can understand you being sad over a career plan not going right but it should be more disappointment than sadness. Life is always throwing curve balls at us and it's how we deal with them that matters, and this may just send you in a better direction. You have so much more than many people do so I would concentrate on what is "right" in your life instead of what isn't. We often have no clue as to why we are having such bad anxiety and therapy can help recall something that happened a long time ago that you thought had been neatly tucked away. But when this is the case it will rear its ugly head at some point and it can be in the form of anxiety and/or depression. But it sounds like you put a lot of pressure on yourself when it comes to your career and that may well be the culprit. It's great to be ambitious but if it's gotten to the point of creating severe anxiety then you need to take a close look at the whole picture. Look at your good days and try to see what was different that you had no anxiety. You may have deadlines or quotas to make and that can cause a lot of anxiety. Are you mostly anxious at work? A road block was put in the way of your career, we don't just stop but find another way or career. I hope this helps and wish you all the best.
It can be an organic thing with no triggers or causes at all. We all do have bad, good, and ugly (worst of worst) days. That's human. It's a disorder when the bad days are far outnumbering the good and we aren't coping very well. Then there is help out there. Where are you at in that regard? Are you at the point of needing help for this?
Actually, most sufferers of chronic anxiety and depression don't have any cause they can point to. Also realize, you can take a million people, expose them to the same situation, and you'll get a million different reactions. We're all snowflakes. Now, sometimes there is something back there you've repressed or just don't really remember that you can find if you do ten years of therapy, but truly, most of us don't really have something that caused it. A career choice not going according to plan -- whose does? Sadness isn't a mental illness, it's just a reaction to bad things happening that goes away. Sometimes a career choice going bad turns into a better choice. That's just life, not illness. If those kinds of things trigger something so severe as to be diagnosed as chronic anxiety or depression it was because it was already there.