Father Christmas
The best advise comes from fathers. They are able to communicate very sophisticated things with very simple phrases. In this scenario, believing in oneself for 10 seconds seems far more rational than believing in Santa. The only proof you had that Santa was real were the gifts under the tree. Recognize your accomplishments and let that be all the proof you need to believe in yourself.
A Good Reminder
Everyone has a unique life path. Even if they come a few years later than intended, accomplishments are still significant. Create a timeline for yourself and measure your achievements against it, as this individual suggests. Be honest about the time and resources you require to complete a task, and don't feel inferior if they differ from those of those around you.