I think the difference between fearless and learning to control your fears is dependent upon maturity, experience, and perspective. Fearless implies an attitude of confidence. That is learned once someone has experienced and matured through facing their fears. Facing fears is a process in which people learn to control their fears. Once you have taken control of your thoughts concerning a fear, then that fear has no more hold on you. Once you change your perspective on a fear, it will no longer have a hold on you. For instance, Four has a fear of heights. However, his perspective of climbing a ferris wheel to protect Tris from falling kept his fear from paralyzing him, so he was able to control his fear. Tris running back into Dauntless headquarters when everyone was still under the attack serum was fearless, because she faced the fear head on with confidence that she was going to accomplish a goal regardless of the circumstances. Her accomplishing her goal of stopping the attack, through facing her fear of being killed by Four was accomplished through her changing her perspective on the situation, and figuring out how to save herself and Four, thus creating bravery on her part. Therefore I think one becomes fearless through learning how to control their fears through changing their perspective.