Unrelated, but I found that (uni) students find it easier to remember what conCAVE and conVex mean by recalling the shape of an entrance to a 'cave' and the shape of letter 'v'. Visual cues are very useful, even if you do not suffer from dyslexia.
For me it was number 7. I fought with it for a number of years. I went on summer break one year around 8 or 9, came back and all my 7's were backwards again. Thing is I couldn't see it till the teacher pointed it out.
If I have an external visual queue I can use, I'm good to go. That's what made the >< thing work. I got the intersection/union symbols down because union happens to both look like and start with a U.