Such fluctuations in mood are common especially when one is under stress. It is better to consult a mental health professional for better management of the problem.

Pls cry alone in a room. This is called cry therapy,after crying u ll become normal  the benifts of cry therapy I have put 5 yrs ago iny blog go through and find its benefits http://mindcounselling.blogspot.in/p/dr-sunderam.html?m=1

Coming out of that vicious circle is bit challenging. This can be handled by understanding your strengths and weaknesses. Many a times,  we tend to cling to our weaknesses instead of appreciating our strengths.

I can totally understand what you are feeling. There are times when thoughts over power is and we feel slightly negative at such a time we try and fight with the thoughts run after them and end up giving extra focus I know it's easier said than done, but with treatment you will be able to distance yourself and gain back positive control. So I strongly suggest you visit a Psychologist and if need be you can be referred to a Psychiatrist. Start this you will feel better soon. We are there to help you can also contact me for further queries or treatment.