To overcome self-doubt we need confidence-inspiring objects of meditation. These focus on the positive, not the negative, sides of ourselves, because when a person has self-doubt there's a tendency for them to select negative ideas or memories of themselves and form it into a solid self image.
These people focus on what they didn't do well enough, when they failed, were rejected, believing in others' blame, etc.
So in order to balance this we have to look at the other side of ourselves so that a beneficial doubt arises, that is, a doubt that our created negative self image is actually true.
One way to do this is to put more time into contemplating our good past Kamma and our Generosity. Remember when we did beneficial things in the past rather than only thinking about things we didn't do well enough, or we did unskillfully.
Another way to overcome self-doubt is to reflect on the Buddha, the Dhamma and the Sangha. This gives confidence in our inner potential. Due to our conditioning, there's a tendency to always think of what we lack and not our strengths.
We seek our happiness by wanting others to approve of us rather then approving of ourselves. By contemplating the Buddha, the Dhamma and the Sangha, it helps us realize that the human potential is vast and that we have that potential.
Cultivating the unselfish emotion of Sympathetic Joy is also a very useful meditation.
Sometimes self-doubt arises due to a tendency to compare yourself with other people who you consider superior to yourself resulting in inferior feelings and self hatred. This can feed jealousy which can also form into aversion to them
So learning how to do Sympathetic Joy meditation at appropriate times is very helpful. Looking at your good qualities, learning how to take joy with your good qualities, learning how to take joy with the good qualities of others, learning how to rejoice in an unselfish way with yourself and with others.
Self-doubt is not something that usually goes away very quickly and so we need to have quite a deal of patience. Sometimes it arises due to excessive idealism, perfectionism, always comparing oneself with the perfect and coming out unfavorably when compared to that.
Learning how to appreciate whatever good qualities we have in the present, together with learning how to have Compassion for ourselves when we see negative qualities, may help in balancing the tendency to freeze them into a solid static self image.
Learning how to forgive oneself and see that we're a changing entity rather than a fixed idea is also very helpful. Understanding we are a changing entity rather than a fixed idea helps us in forgiving ourselves.
Our views, feelings, thoughts, etc. are changing and if we are practicing, we are letting go of some of the ignorance that caused us to do unskillful things. We can forgive the person we used to be when we didn't do things so well and realize that the person we are in the present is different, has different views, has different ideas about things.
So having compassion for and forgiving the people we used to be helps us to resolve the past so that memories or images of ourselves don't influence us as much.
Developing the unselfish emotions; Compassion, Lovingkindness, Sympathetic Joy and Equanimity helps us develop an emotional maturity and confidence this helps to overcome self-doubt.