As a matter of fact, this is one of the few times where my advice won't include "talk to him." You said you have, and I'll trust that you have fully explored that particular avenue of solution. Let me start be trying to explain what I see going on before I get you any of my tips about how to proceed. First of all, the problem with your grades is actually quite a common one when girls are in relationships with guys were they outperform them in school, or really any activity. It's not a false stereotype that guys are intensely competitive with extremely weak egos. Your boyfriend likely has an image of himself as more than he is in his head, and like most guys, he is trying to find a girl who validates that image. You obviously are trying to, but no matter how often you congratulate him, all he sees is that you have outperformed him.
I sympathize with him. I consider myself to be a very secure person, but when I'm in a class with a friend of mine who is comparably intelligent, I find myself in constant unspoken competition with her. If I don't score as well as she does, I have to make excuses, and that is what he is doing when he is putting you down.
That doesn't make it right, of course. It is still very wrong, but that's why he is doing it. In fact, very little of his described behavior seems acceptable to me. In my experience, when a girlfriend makes a post like the one you've made, one of two things is going through her head. Either she is looking for reassurance that "love will prevail" or she wants someone to absolve her of any guilt she might have over ending the relationship.
If you are looking for someone to tell you that "being in love" is enough to make a relationship work, that's not me. I've been married for two years now and can state categorically that if we had expected our love to see us through hard time, we would already be divorced. "Being in love" is not going to make your relationship healthy.
If you're looking for someone to tell you to break up with him or to make you feel better about thinking about it, I won't do that either. I try not to make big relationship decisions for people. What I will tell you is that you have not described a healthy relationship. You need to understand that youth is for education. You're too young to have put all your eggs in this basket. There are a thousand other guys in the world that are as good or better for you than the guy you are with now. Date while you're young. Longing for that "new relationship" feeling is totally natural and you should, to a degree, embrace it while you're young.
Here's the talking advice: talk to your friends. They know your relationship better than I do (or anyone on LiveWire). If they think it's time to move on, then maybe you should listen to them. Friends often have a clearer view of what is going on in our lives than even we do. That's why we believe in peer support on LiveWire. You have a community of friends in your life. Let them help you.