Sell somebody/yourself short
Definition 1 of 2
Pro Tip 1/3Positive Reinforcement
Use it to encourage someone to recognize their true worth or abilities. 
You ace every project you take on—don't sell yourself short!
Pro Tip 2/3Self-Confidence
Implies that someone has more ability or positive qualities than they realize. 
You speak four languages fluently, so don't sell yourself short during the interview.
Pro Tip 3/3Motivational Speech
Commonly used in pep talks to inspire better self-esteem. 
You've trained hard for this marathon—don't sell yourself short now!