Sure of yourself
Definition 1 of 1
Pro Tip 1/3Context Matters
Use 'sure of yourself' when the situation involves personal confidence, not when talking about being certain of facts or information. 
After practicing for weeks, she was sure of herself during the performance.
Pro Tip 2/3Positive Connotation
The phrase 'sure of yourself' often implies positivity like self-confidence, but be aware it can sometimes be perceived as arrogance. 
His sureness of himself was inspiring, though some found it overbearing.
Pro Tip 3/3Personal Growth
Often used to describe someone who has grown or developed, gaining confidence in specific areas or generally. 
She used to be shy, but now she's really sure of herself and speaks up.