I think we all have self-doubts. The challenge is to not let them become the whole of our self-definition. It can be hard, especially when our mood is down, to not believe the worst about ourselves. Reminders that the doubts only represent one small part of ourselves can be helpful. Sometimes it can help to spend some time everyday rewriting lists of all the experiences we've had that counter those nagging doubts. It's so easy to forget them, otherwise.