It’s really important for me to hear your opinions

No one can or should tell you how to feel about anything.

Your feelings are yours. Own them.

And, apparently, also study harder next time.

Do you know WHY you got a mediocre grade on your assignment?

The best source for this - is your professor.

Make an appointment - and let your professor know you are saddened by this grade and want to improve. Then ask what you did right and what you did wrong...ask how you can improve and any thoughts and suggestions.

I can understand being disappointed but one way to look at this is an opportunity to improve. I often think that sometimes it is actually helpful to sometimes not perform to what you set as a goal or what you expect. It gives you an opportunity to see how you can improve upon your work - get another perspective (your professor's in this case) and be an even better writer than you were before.

Also it could simply be a misunderstanding of what you think the professor expects from your essay --- it could simply be as much that your professor expects something different than you thought.

And also on the positive - seeing that you are saddened just shows how much you care about doing well - that is definitely a positive.