Is it true that engaging in sports and learning increases self-confidence, while masturbation decreases it, and is this religiously sound, considering that self-confidence may not be obligatory or permissible?

The statement that self-confidence is neither obligatory nor permissible has no basis, and the conclusion built upon it and the effects of a bad habit is flawed. Shariah rulings have known sources, and a bad habit is Islamically forbidden.