'gain self-confidence' is correct and usable in written English.
You can use it when you are talking about someone increasing their self-confidence. For example: "She took a series of confidence building workshops in order to gain self-confidence."
The training helped Muna to gain self-confidence and the skills to assist others, "I am confident about myself, I can even speak on national TV.
Such experiences help students not only to develop critical intellectual skills and traits, but also to gain self-confidence, evaluate their progress, and make personal and professional choices.
Parents' priorities included having their children learn to interact well with others; show respect, affection, and honesty; and gain self-confidence and positive self-image.
As a way of helping Tiryese deal with difficult emotions, gain self-confidence, learn discipline and stay busy after school, Mr. Fredericksen is enrolling him in a karate program.
The course helps students gain self-confidence in themselves as field biologists and gives them a commitment to teamwork, said Professor Emeritus Harry Greene, who has taught field courses for 40 years.
Today, in lieu of a lesson plan, we've invited Larry Ferlazzo, a teacher and author, to write a guest post that brings together a number of recent Times and Learning Network pieces about motivation — and shows practical ways teachers can use them to help students learn to exert self-control, gain self-confidence and motivate themselves to learn and achieve.
Meanwhile April gains self-confidence.
"I gained self-confidence," he said.
All of them seem to have gained self-confidence and self-worth.
All those hopes for children's gaining self-confidence or friends or independence!
With a more open private-sector economy, more women are now working for themselves, starting businesses, earning money and gaining self-confidence.
