I have no idea what it is called. Opposites always frustrated me and I mix them up. Sometimes I mix up the concept and if I do that it sticks... sometimes I just mix up the words.

I often say up when I mean down, and I say next year instead of last year, yesterday instead of tomorrow. I think the words are stored so close to each others in my head that I hit the wrong one a lot. I practice a lot with concepts, like really remember which is the Arctic and which is Antarctica, stuff like that.