Comparing myself to others: How do I stop?

The title says it all. I've had this huge problem of constantly comparing myself to other people since I can possibly remember. No matter how hard I try to reject this feeling, no matter how much effort I put in the things I do, there is always someone better and it makes me feel like and absolute failure. I need serous help, because always feeling inferior stops me from being happy and successful in life. Recently, an even bigger problem has occurred. I've been in a relationship for little less than a year now. My significant other is incredibly smart, honestly the most intelligent, well organised, studious person I have ever met. But being the way I am, I can't help being a bit envious, even though I know it is absolutely wrong. We attend the same school and they are well known (not only for his academic performance). What I have noticed and starts to bother me is that when teachers talk to me and find out that this person and I are in a relationship, they immediately start to praise them and their incomparable smartness. This makes me feel genuinely sad, because I feel like people don't see me, but X's partner and feel the urge to praise them in every possible way. I have a good academic performance too and I am proud of it. But recently a teacher we have in common said these specific words in front of my class: "We all know X is in a relationship with a dumb person", and yes, I am the dumb person. He was joking, but I'm pretty sure there was a bit of truth in what he said. At the time I brushed that comment off with a stupid comeback and didn't really stand up against it. Being the way I am, I regret it. The point of it all is: I feel a constant competition between my partner and I, almost as a responsibility I didn't sign for. I am afraid that when people look at us, they'll see me as the stupid one. I have been called stupid my whole life, especially as a child, and I am very angry because of this because I know my worth. But sometimes, teachers and people make me forget that. At this point, I don't know what to do. I talked to my parter about this, but the discussion wasn't satisfying at all and ended up being one of the most selfish things they have ever said. I need help because I'm not happy but I don't want to leave them because I think it would be stupid to end a relationship because of school marks. I don't know what to do.