Negative thoughts – where do they start?

I have a negative thought that I am weak and a fake – I was telling a co-worker about my coaching session where we revealed these thoughts about myself – my co-worker mentioned she felt like something must have happened or been done to me in my past that was the initial trigger for these thoughts – otherwise why would they be there. I couldn’t disagree…do you think that’s they case? And if so – is it helpful to try to figure out what that event was so I can change my thinking about it? And if it would be helpful – how do I even go about trying to figure out what it was?