My first B grade

Hi! I am a high achiever and take great pride in any project or assignment I take on. I have been pursuing my masters in organizational leadership since 2017 and I am just ending my very last class. We turned in a final thesis last week which was the culmination of everything we learned throughout the program. It was the first B I earned in the program. I knew this professor was hard (he seems to grade subjectively) so I did all the “things” I could to create my best work (writing center, draft feedback, multiple proofreads, etc.) I feel like I should be celebrating the end of this program, but instead, I am left with a bad taste in my mouth. I feel like I failed despite all of the careful and thorough effort I put in and that makes me feel less than. I also am trying to watch my thoughts recognizing they are “like a toddler with a knife” yet I feel so deflated. How can I break this down in a way that will shift my thinking? Thanks!