I need to learn what?

I do this sometimes. I think it's because I've learnt the two characters as a word and not individual characters, and automatically pull one of them up from memory, but the wrong one. I tend to try to catch myself when I do that and consciously separate the characters in my mind after that, so I build a distinct reading-memory for each.

I can't think of a good example now though. But it normally happens if I rarely encounter the two characters outside of that particular word that I know

I was once reading something in Japanese and came across 今日 (kyō) and read it as 今天 (jīntiān), That one threw me off for a second. I switched characters and languages, but somehow kept the register (they're both less formal words for "today"). I guess my brain thought that was the most important thing. I don't know.

The same thing happens to me but with different characters. It's probably best I don't mention them so that that you don't "learn" my mistake by accident!