Self doubts often arise when we over-think things and compare ourselves with others. Doubt becomes overbearing by our own over-thinking, over analysis and self criticism. Be easy on yourself, it is ok to lose some we can learn from our mistake. Stop over analyzing and over thinking, this is a waste of both time and energy, instead focus on the goals and the ways to achieve them. We know your friends are all working, getting married, having kids and all this creates doubt. Try to see the lighter side of things and use humor in generous measure to see things in different perspective.
Realize that you are a unique individual and everyone is different. Do not get bogged down by others criticisms, you can always work on your flaws to be a better person. But focus on your positive qualities. Seek support from family and friends who are always ready to pep and boost your confidence, who encourage you to see the brighter side of life, events and self.