Focus more on healing the people who are fighting. Pharah was fighting, you healed her for a split second and dove away to top off an ally who wasn't fighting. In that time Pharah died. You rezzed and the same thing happened and she died again. You could have stood on the platform to continue healing Pharah without taking heat from the enemies on the point.
I'm not saying that everything here is your fault, it's certainly not, but there were things you could have done that might have swung the fight