A common tactic is to reassure our self-doubting friends about how great they are. Increasingly I believe this does not work. Often the doubt is there in the first place as a protective measure against failure and disappointment. If you talk the friend up, the friend in turn has to doubt herself more.
If a friend doubts herself, often it is better to respond by doubting that same friend all the more.
It is as if the friend needs a fixed amount of doubt. The more you carry that load, the less the friend has to doubt herself to reach the fixed level of doubt needed to produce preemptive protection and reassurance.