What is the most powerful curse?

The most powerful curse is the most insidious... the speaking of negative thought.

If you say it aloud, your mind believes it on some level. What your mind believes it can manifest...

Therefore if you say, "I can't do that." You can't. If you say, "I can't win." You can't. Anything negative you say, has more power than all the spells and curses anyone else can throw at you.