Graciousness in defeat
You will lose. How you approach that loss though, is everything. Learn from it. Find out why and how. Try and evaluate your strengths, weaknesses and points you had the ability to change but didn't. This is how you learn and become a better player.
The person that beat you, no matter the circumstance, did so because they were better than you in that moment. Let them have that moment, acknowledge their victory and move on.
As a white belt, you will lose frequently. Don't lose heart. With consistent training, you will get better. Losing doesn't mean you are getting any worse, it just shows you new obstacles to overcome. Ask your training partners how they managed to get x, y or z and try to not make the same mistake again.