You're Reinhardt.
You're protecting teammates with your shield. Suddenly you charge far away from your allies in an attempt to kill an enemy. You fail to kill anyone, swing your hammer a few times at enemies who saw you coming from a mile away, and die within seconds. You left your teammates to die in a 5v6 in which they don't have a shield, but the enemy team does. This repeats several times and the enemy team eventually wins.

What went wrong?