I believe that true, sustainable self-belief must be founded upon strong evidence to suggest that you are worth believing in. Yet sometimes, there is no other option than to blindly and stupidly believe, to simply trust oneself without reason, regardless of the evidence to suggest its futility.
Unnatural as it may feel, blindly believe that you will, and keep doing this until belief comes more naturally, through your experience, both in practice and in achievement.
Otherwise, you will remain in turmoil with yourself, awaiting confirmation from your experience that there is reason to believe, or committing successive attempts yet repeatedly failing due to a lack of self-belief, further fuelling reason not to trust yourself in the future.
To break from this cycle (which would suggest that building self-belief is impossible for those who have little in the first place) you must temporarily trust the process, as futile as this may seem in the worst of times.