Self, I too easily fall into a cycle where I just continue to criticise and feel worse and worse about myself. While outside criticism can contribute to this it is the inner criticism that continues the cycle.
It's interesting that you say that because I'm going through a similar situation, where someone else's criticism of me rattles my foundation or what I originally thought about myself and it just goes downhill from there.
I've fallen into this cycle many times. It was the cause of a case of minor depression in primary school and I had some moments in high school. I can't give you any advice, each time I pulled myself out of it was different, sometimes it was time, sometimes people.
Hmmm unfortunately, I have neither time nor people to pull me out of this doubt and it would seem that I have to climb out of it myself in order to finish this assignment on time. It just bugs me though, because I don't know if my "critic" was being honest or just being an asshole. I mean, is there a right way to offer criticism? Perhaps I am just being overly sensitive.
There is a right way to offer criticism, but I'm afraid not many people practice it. Its called the hamburger technique, sandwiching the comments on what is "bad" between two things which are good. We get taught it a lot doing a teaching degree. A lot of people think that's a weak way of doing things though and don't understand its importance.
For those things I don't have much time to get out of moods it tends to help me to write it all down and then almost immediately to sleep it off. I guess the subconscious mind deals with it. I tend to also benefit by writing it somewhere that someone will read it.