Most of us struggle with self-doubts about whether we can achieve our goals and projects. What makes the difference between success and failure is not whether we have self-doubts, but what we do when the doubts creep in.
What can you do is to go back to these two attributes: Positive Mental Attitude and Health Self-esteem. Accept your thoughts about the self-doubts, and reframe them in a more positive way i.e. as an opportunity to test yourself and your capacity to move forward knowing that there may be obstacles along the way.
Take a step backward and review the specific goal or project that is giving you some concerns. However, be careful not to give yourself an excuse to procrastinate. After a careful re-evaluation, you need to take decisive action, either abandon the project as being unrealistic for the moment, or going ahead facing your doubts and fears squarely.
So take decisive actions, and learn from your mistakes or successes. This will build your self-confidence, and will lead you to a more fulfilled life.