Well, first of all, you know how SD works in stats and good for you. That alone tells me you're pretty smart. Your T may think you're smart but has never said it. You said your self that you feel frustrated when you receive grades like B and it seems like your T knows this. She probably doesn't want to focus too much on this for that reason. I think your T is using a 'can't see the forest through the trees' approach.
I'm not sure if your in high school, college, grad school, etc but your receiving an education and really grades don't matter. The overarching goal is for you to gain knowledge and grades are the small stepping stones to that final big goal.
However, you do have big expectations of your self. I truly think you're smart but often thinks get in the way of doing well in classes. Issues such as mental illness, physical illness, if your tired or hungry, your mood that day, etc can change your grade.
Hope I helped.